U.S. patent application number 12/088037 was filed with the patent office on 2009-10-29 for production method of microporous polyethylene membrane and battery separator.
This patent application is currently assigned to Tonen Chemical Corporation. Invention is credited to Shintaro Kikuchi, Kotaro Kimishima, Kotaro Takita.
Application Number | 20090269672 12/088037 |
Document ID | / |
Family ID | 37899713 |
Filed Date | 2009-10-29 |
United States Patent
Application |
20090269672 |
Kind Code |
A1 |
Takita; Kotaro ; et
al. |
October 29, 2009 |
PRODUCTION METHOD OF MICROPOROUS POLYETHYLENE MEMBRANE AND BATTERY
SEPARATOR
Abstract
A microporous polyethylene membrane having well-balanced
permeability, mechanical properties, heat shrinkage resistance,
compression resistance, electrolytic solution absorbability,
shutdown properties and meltdown properties, with an average pore
diameter changing in a thickness direction is produced by
melt-blending a polyethylene resin and a membrane-forming solvent
to prepare a solution A having a resin concentration of 25 to 50%
by mass and a solution B having a resin concentration of 10 to 30%
by mass, the resin concentration in the solution A being higher
than that in the solution B, (a) simultaneously extruding the resin
solutions A and B through a die, cooling the resultant extrudate to
provide a gel-like sheet in which the resin solutions A and B are
laminated, and removing the membrane-forming solvent from the
gel-like sheet, or (b) extruding the resin solutions A and B
through separate dies, removing the membrane-forming solvent from
the resultant gel-like sheets A and B to form microporous
polyethylene membranes A and B, and alternately laminating the
microporous polyethylene membranes A and B, while easily
controlling the average pore diameter distribution in the
microporous polyethylene membrane in a thickness direction.
Inventors: |
Takita; Kotaro;
(Nasushiobara-shi, JP) ; Kikuchi; Shintaro;
(Saitama-shi, JP) ; Kimishima; Kotaro;
(Yokohama-shi, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W., SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
Tonen Chemical Corporation
Minato-ku
JP
|
Family ID: |
37899713 |
Appl. No.: |
12/088037 |
Filed: |
September 27, 2006 |
PCT Filed: |
September 27, 2006 |
PCT NO: |
PCT/JP2006/319208 |
371 Date: |
March 25, 2008 |
Current U.S.
Class: |
429/254 ;
264/51 |
Current CPC
Class: |
B01D 71/26 20130101;
B01D 2325/34 20130101; H01M 50/44 20210101; B01D 67/002 20130101;
B32B 27/32 20130101; B32B 5/18 20130101; B01D 69/12 20130101; Y02E
60/10 20130101; B01D 2325/22 20130101; H01M 50/403 20210101; H01G
9/02 20130101; B01D 2323/12 20130101; H01M 10/0565 20130101; H01M
50/411 20210101 |
Class at
Publication: |
429/254 ;
264/51 |
International
Class: |
H01M 2/16 20060101
H01M002/16; B29C 44/50 20060101 B29C044/50 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 28, 2005 |
JP |
2005-283030 |
Claims
1. A method for producing a microporous polyethylene membrane
having an average pore diameter changing in a thickness direction,
comprising the steps of melt-blending at least a polyethylene resin
and a membrane-forming solvent to prepare a polyethylene resin
solution A having a resin concentration of 25 to 50% by mass and a
polyethylene resin solution B having a resin concentration of 10 to
30% by mass, the resin concentration in the polyethylene resin
solution A being higher than that in the polyethylene resin
solution B; simultaneously extruding the polyethylene resin
solutions A and B through a die; cooling the resultant laminate
extrudate to provide a gel-like sheet; and removing the
membrane-forming solvent from the gel-like sheet.
2. A method for producing a microporous polyethylene membrane
having an average pore diameter changing in a thickness direction,
comprising the steps of melt-blending at least a polyethylene resin
and a membrane-forming solvent to prepare a polyethylene resin
solution A having a resin concentration of 25 to 50% by mass and a
polyethylene resin solution B having a resin concentration of 10 to
30% by mass, the resin concentration in the polyethylene resin
solution A being higher than that in the polyethylene resin
solution B; extruding the polyethylene resin solutions A and B
through separate dies; cooling the resultant extrudates to provide
gel-like sheets A and B; removing the membrane-forming solvent from
the gel-like sheets A and B to form microporous polyethylene
membranes A and B; and alternately laminating the microporous
polyethylene membranes A and B.
3. The method for producing a microporous polyethylene membrane
according to claim 1, wherein the resin concentration difference
between the polyethylene resin solutions A and B is 5% or more by
mass.
4. The method for producing a microporous polyethylene membrane
according to claim 1, wherein the polyethylene resin comprises a
polyethylene composition comprising ultra-high-molecular-weight
polyethylene having a mass-average molecular weight of
7.times.10.sup.5 or more, and high-density polyethylene having a
mass-average molecular weight of 1.times.10.sup.4 or more and less
than 5.times.10.sup.5.
5. The method for producing a microporous polyethylene membrane
according to claim 4, wherein the polyethylene resin is a
composition comprising the polyethylene composition, and a
heat-resistant resin having a melting point or glass transition
temperature of 150.degree. C. or higher.
6. The method for producing a microporous polyethylene membrane
according to claim 5, wherein the heat-resistant resin is
polypropylene or polybutylene terephthalate.
7. A battery separator formed by the microporous polyethylene
membrane produced by the method recited in claim 1.
8. The method for producing a microporous polyethylene membrane
according to claim 2, wherein the resin concentration difference
between said polyethylene resin solutions A and B is 5% or more by
mass.
9. The method for producing a microporous polyethylene membrane
according to claim 2, wherein said polyethylene resin comprises a
polyethylene composition comprising ultra-high-molecular-weight
polyethylene having a mass-average molecular weight of
7.times.10.sup.5 or more, and high-density polyethylene having a
mass-average molecular weight of 1.times.10.sup.4 or more and less
than 5.times.10.sup.5.
10. A battery separator formed by the microporous polyethylene
membrane produced by the method recited in claim 2.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a method for producing a
microporous polyethylene membrane, and a battery separator,
particularly to a method for producing a microporous polyethylene
membrane with an average pore diameter changing in a thickness
direction, and a battery separator.
BACKGROUND OF THE INVENTION
[0002] Microporous polyolefin membranes are widely used in
separators for lithium batteries, etc., electrolytic capacitor
separators, various filters, etc. When the microporous polyolefin
membranes are used as battery separators, their performance largely
affects the performance, productivity and safety of batteries.
Particularly lithium ion battery separators are required to have
excellent mechanical properties and permeability, as well as
shutdown property, a function of closing pores to stop a battery
reaction at the time of abnormal heat generation, thereby
preventing the heat generation, ignition and explosion of the
battery, which can be caused by the short-circuiting of external
circuits, overcharge, etc.; heat shrinkage resistance, a function
of keeping a separator shape to avoid a direct reaction between a
cathode material and an anode material even when becoming high
temperatures, etc.
[0003] Recently gaining importance as separator characteristics are
not only permeability, mechanical strength, heat shrinkage
resistance and thermal properties (shutdown properties and meltdown
properties), but also battery life properties such as cycle
properties (properties concerning battery capacity when used
repeatedly), and battery productivity such as electrolytic solution
absorbability. A lithium ion battery electrode expands by the
intrusion of lithium when charged, and shrinks by the departure of
lithium when discharged, an expansion ratio when charged tending to
become larger as recent increase in the capacity of batteries.
Because a separator is compressed when the electrode expands, the
separator is required to suffer only small permeability variation
by compression to have excellent cycle properties. To that end,
there are (i) a technology of providing a separator with a gradient
structure comprising a coarse-structure layer having a relatively
large average pore diameter, which undergoes large deformation with
small air permeability change when compressed, and a
dense-structure layer having a relatively small average pore
diameter, which undergoes large air permeability change with small
deformation when compressed, the coarse-structure layer absorbing
the expansion of an electrode and holding permeability; and (ii) a
technology of making the deformation of the entire separator small
to prevent a pore structure from being broken. These technologies
are properly selected depending on the properties of
electrodes.
[0004] To improve the electrolytic solution absorbability, it is
effective to provide a large pore size to the separator surface.
Also, to prevent by-products generated by the repetition of
charge/discharge cycles from clogging the separator, the separator
is required to have a large pore size on the surface. However, to
secure the mechanical strength, a dense layer is needed. Thus, to
satisfy both requirements of high electrolytic solution
absorbability and high mechanical strength, the separator is
desired to have a coarse-structure layer having a relatively large
average pore diameter on at least one surface, in addition to a
dense-structure layer.
[0005] Liquid filters are desired to have higher filtering
performance, and for this purpose, microporous membranes should
have smaller pores. However, to avoid decrease in the filtering
efficiency, the microporous membrane should not deteriorate liquid
permeability. To meet both requirements of high filtering
performance and high liquid permeability, the liquid filters
desirably have the above gradient structure. Specifically, the
balance of the filtering performance and the liquid permeability
can be controlled by constituting the microporous membrane by a
dense-structure layer as a support layer and a coarse-structure
layer as a filtering layer, and adjusting the thickness ratio of
the dense-structure layer to the coarse-structure layer.
[0006] A microporous polyolefin membrane, JP 2000-212323 A
discloses a microporous polyolefin membrane different between the
internal structure and the surface structure to have excellent pin
puncture strength and porosity, which has an average pore size of
0.01 to 0.2 .mu.m, at least one surface thereof having an average
pore size of 0.5 to 2 .mu.m. This microporous polyolefin membrane
is produced by (i) melt-blending a polyolefin and a plasticizer to
prepare a polyolefin solution, extruding and cooling the polyolefin
solution to form a sheet, stretching the sheet, and then extracting
the plasticizer from the stretched sheet to form a microporous
membrane 1 having an average pore size of 0.5 to 2 .mu.m on at
least one surface, (ii) further stretching the microporous membrane
1 while heating to form a microporous membrane 2 having an average
pore size of 0.01 .mu.m or more, and (iii) laminating the
microporous membranes 1 and 2.
[0007] JP 2003-105123 A discloses a microporous polyolefin membrane
comprising polyethylene having a mass-average molecular weight (Mw)
of 5.times.10.sup.5 or more as an indispensable component, and
having an average pore size change in a thickness direction,
wherein at least one surface thereof is larger in average pore size
than inside, or wherein one surface being larger in average pore
size than the other surface, so that the microporous polyolefin
membrane has excellent pin puncture strength, heat shrinkage
resistance and permeability. This microporous polyolefin membrane
is produced by (a) melt-blending a polyolefin comprising
polyethylene having Mw of 5.times.10.sup.5 or more as an
indispensable component with a membrane-forming solvent, extruding
the resultant melt blend through a die, cooling the extruded melt
blend to provide a gel-like sheet, biaxially stretching the
gel-like sheet with a temperature distribution in a thickness
direction, removing the solvent from the stretched gel-like sheet,
stretching the resultant membrane in at least one direction, and
then heat-treating the membrane at a temperature in a range of the
crystal dispersion temperature of the polyolefin or higher and
lower than the melting point of the polyolefin to form a
microporous membrane (i), (b) stretching the above gel-like sheet
in at least one direction at a temperature lower than the crystal
dispersion temperature of the polyolefin, and then stretching the
gel-like sheet in at least one direction at a temperature in a
range of the crystal dispersion temperature of the polyolefin or
higher and lower than the melting point of the polyolefin, and
further removing the solvent from the stretched membrane to form a
microporous membrane (ii), and (c) laminating the microporous
membranes (i) and (ii). In the microporous membranes of the above
references, however, layers having different average pore sizes are
formed under different stretching conditions, but not under
different melt blend concentrations. Accordingly, they do not
necessarily have well-balanced permeability, mechanical properties,
heat shrinkage resistance, compression resistance, shutdown
properties and meltdown properties.
OBJECT OF THE INVENTION
[0008] Accordingly, an object of this invention is to provide a
method for producing a microporous polyethylene membrane having
well-balanced permeability, mechanical properties, heat shrinkage
resistance, compression resistance, electrolytic solution
absorbability, shutdown properties and meltdown properties, with an
average pore diameter changing in a thickness direction, while
easily controlling the average pore diameter distribution in a
thickness direction, and a battery separator.
DISCLOSURE OF THE INVENTION
[0009] As a result of intense research in view of the above object,
the inventors have found that a microporous polyethylene membrane
having well-balanced permeability, mechanical properties, heat
shrinkage resistance, compression resistance, electrolytic solution
absorbability, shutdown properties and meltdown properties, with an
average pore diameter changing in a thickness direction can be
produced by melt-blending a polyethylene resin and a
membrane-forming solvent to prepare a solution A having a resin
concentration of 25 to 50% by mass and a solution B having a resin
concentration of 10 to 30% by mass, the resin concentration in the
solution A being higher than that in the solution B, (a)
simultaneously extruding the resin solutions A and B through a die,
cooling the resultant extrudate to provide a gel-like sheet in
which the resin solutions A and B are laminated, and removing the
membrane-forming solvent from the gel-like sheet, or (b) extruding
the resin solutions A and B through separate dies, removing the
membrane-forming solvent from the resultant gel-like sheets A and B
to form microporous polyethylene membranes A and B, and alternately
laminating the microporous polyethylene membranes A and B, the
average pore diameter distribution in the microporous polyethylene
membrane in a thickness direction being easily controlled. This
invention has been completed based on such finding.
[0010] Thus, the first method of this invention for producing a
microporous polyethylene membrane having an average pore diameter
changing in a thickness direction comprises the steps of
melt-blending at least a polyethylene resin and a membrane-forming
solvent to prepare a polyethylene resin solution A having a resin
concentration of 25 to 50% by mass and a polyethylene resin
solution B having a resin concentration of 10 to 30% by mass, the
resin concentration in the polyethylene resin solution A being
higher than that in the polyethylene resin solution B;
simultaneously extruding the polyethylene resin solutions A and B
through a die; cooling the resultant laminate extrudate to provide
a gel-like sheet; and removing the membrane-forming solvent from
the gel-like sheet.
[0011] The second method of this invention for producing a
microporous polyethylene membrane having an average pore diameter
changing in a thickness direction comprises the steps of
melt-blending at least a polyethylene resin and a membrane-forming
solvent to prepare a polyethylene resin solution A having a resin
concentration of 25 to 50% by mass and a polyethylene resin
solution B having a resin concentration of 10 to 30% by mass, the
resin concentration in the polyethylene resin solution A being
higher than that in the polyethylene resin solution B; extruding
the polyethylene resin solutions A and B through separate dies;
cooling the resultant extrudates to provide gel-like sheets A and
B; removing the membrane-forming solvent from the gel-like sheets A
and B to form microporous polyethylene membranes A and B; and
alternately laminating the microporous polyethylene membranes A and
B.
[0012] The resin concentration difference between the polyethylene
resin solutions A and B is preferably 5% or more by mass, more
preferably 10% or more by mass. The polyethylene resin preferably
comprises a polyethylene composition comprising
ultra-high-molecular-weight polyethylene having a mass-average
molecular weight of 7.times.10.sup.5 or more, and high-density
polyethylene having a mass-average molecular weight of
1.times.10.sup.4 or more and less than 5.times.10.sup.5. The
polyethylene resin can comprise a heat-resistant resin having a
melting point or glass transition temperature of 150.degree. C. or
higher. The heat-resistant resin is preferably polypropylene or
polybutylene terephthalate.
[0013] The battery separator of this invention is produced by the
above first or second method.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] [1] Polyethylene Resin
[0015] The polyethylene resin forming the microporous polyethylene
membrane, which can be called simply as "microporous membrane"
below, is (a) ultra-high-molecular-weight polyethylene, (b)
polyethylene other than the ultra-high-molecular-weight
polyethylene, (c) a mixture of the ultra-high-molecular-weight
polyethylene and the other polyethylene (polyethylene composition),
(d) a mixture of any one of (a) to (c) with a polyolefin other than
polyethylene, polypropylene and polymethylpentene (polyolefin
composition), or (e) a mixture of any one of (a)-(d) with a
heat-resistant resin having a melting point or glass transition
temperature Tg of 150.degree. C. or higher (heat-resistant
polyethylene resin composition). In any case, the mass-average
molecular weight (Mw) of the polyethylene resin is preferably
1.times.10.sup.4 to 1.times.10.sup.7, more preferably
1.times.10.sup.4 to 5.times.10.sup.6, particularly 1.times.10.sup.4
to 4.times.10.sup.6, through not particularly critical. With the
polyethylene resin having Mw of 5.times.10.sup.6 or less, a
microporous layer having a large pore size and high permeability
can be obtained.
[0016] (a) Ultra-High-Molecular-Weight Polyethylene
[0017] The ultra-high-molecular-weight polyethylene has Mw of
7.times.10.sup.5 or more. The ultra-high-molecular-weight
polyethylene can be not only an ethylene homopolymer, but also an
ethylene-.alpha.-olefin copolymer containing a small amount of
another .alpha.-olefin. The other .alpha.-olefins than ethylene are
preferably propylene, butene-1, pentene-1, hexene-1,
4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and
styrene. The Mw of the ultra-high-molecular-weight polyethylene is
preferably 1.times.10.sup.6 to 15.times.10.sup.6, more preferably
1.times.10.sup.6 to 5.times.10.sup.6. Not only one type of
ultra-high-molecular-weight polyethylene, but also a mixture of two
or more ultra-high-molecular-weight polyethylenes can be used. The
mixture can be, for instance, a mixture of two or more
ultra-high-molecular-weight polyethylenes having different Mws.
[0018] (b) Polyethylene Other Than Ultra-High-Molecular-Weight
Polyethylene
[0019] The polyethylene other than the ultra-high-molecular-weight
polyethylene has Mw of 1.times.10.sup.4 or more and less than
5.times.10.sup.5, preferably being at least one selected from the
group consisting of high-density polyethylene, intermediate-density
polyethylene, branched low-density polyethylene and linear
low-density polyethylene, more preferably high-density
polyethylene. The polyethylene having Mw of 1.times.10.sup.4 or
more and less than 5.times.10.sup.5 can be not only an ethylene
homopolymer, but also a copolymer containing a small amount of
another .alpha.-olefin such as propylene, butene-1, hexene-1, etc.
Such copolymers are preferably produced using single-site
catalysts. Not only one type of polyethylene other than the
ultra-high-molecular-weight polyethylene, but also a mixture of two
or more polyethylenes other than the ultra-high-molecular-weight
polyethylene can be used. The mixture can be for instance, a
mixture of two or more high-density polyethylenes having different
Mws, a mixture of similar intermediate-density polyethylenes, a
mixture of similar low-density polyethylenes, etc.
[0020] (c) Polyethylene Composition
[0021] The polyethylene composition is a mixture of
ultra-high-molecular-weight polyethylene having Mw of
7.times.10.sup.5 or more, and the other polyethylene, which is at
least one selected from the group consisting of high-density
polyethylene, intermediate-density polyethylene, branched
low-density polyethylene, and linear low-density polyethylene. The
ultra-high-molecular-weight polyethylene and the other polyethylene
can be the same as described above. The other polyethylene
preferably has Mw of 1.times.10.sup.4 or more and less than
5.times.10.sup.5 The molecular weight distribution [mass-average
molecular weight/number-average molecular weight (Mw/Mn)] of this
polyethylene composition can be easily controlled depending on
applications. The polyethylene composition is preferably a
composition of the above ultra-high-molecular-weight polyethylene
and high-density polyethylene. The content of the
ultra-high-molecular-weight polyethylene in the polyethylene
composition is preferably 1% or more by mass, more preferably 2 to
50% by mass, based on 100% by mass of the entire polyethylene
composition.
[0022] (d) Polyolefin Composition
[0023] The polyolefin composition is a mixture of the
ultra-high-molecular-weight polyethylene, the other polyethylene or
the polyethylene composition, and a polyolefin other than
polyethylene, polypropylene and polymethylpentene. The
ultra-high-molecular-weight polyethylene, the other polyethylene,
and the polyethylene composition can be the same as described
above.
[0024] The polyolefin other than polyethylene, polypropylene and
polymethylpentene can be at least one selected from the group
consisting of polybutene-1, polypentene-1, polyhexene-1,
polyoctene-1, polyvinyl acetate, polymethyl methacrylate,
polystyrene and ethylene-.alpha.-olefin copolymers each having Mw
of 1.times.10.sup.4 to 4.times.10.sup.6, and a polyethylene wax
having Mw of 1.times.10.sup.3 to 1.times.10.sup.4. Polybutene-1,
polypentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate,
polymethyl methacrylate and polystyrene can be not only
homopolymers, but also copolymers containing other .alpha.-olefins.
The content of the polyolefin other than polyethylene,
polypropylene and polymethylpentene is preferably 20% or less by
mass, more preferably 10% or less by mass, based on 100% by mass of
the entire polyolefin composition.
[0025] (e) Heat-Resistant Polyethylene Resin Composition
[0026] The heat-resistant polyethylene resin composition is a
mixture of any one of (a)-(d) above and a heat-resistant resin
having a melting point or glass transition temperature Tg of
150.degree. C. or higher. The heat-resistant resin is preferably a
crystalline resin (including partially crystalline resin) having a
melting point of 150.degree. C. or higher, or an amorphous resin
having Tg of 150.degree. C. or higher. The melting point and Tg can
be measured according to JIS K7121.
[0027] Because a battery separator formed by a microporous membrane
comprising the polyethylene resin containing the heat-resistant
resin has an improved meltdown temperature, batteries are provided
with improved high-temperature storage stability. The
heat-resistant resin is dispersed in the form of spherical or
ellipsoidal fine particles in the homopolymer or composition
described in (a)-(d) above during melt blending. Fibrils of a
polyethylene phase (a phase of the ultra-high-molecular-weight
polyethylene, the other polyethylene or the polyethylene
composition) are cleft with fine, heat-resistant resin particles as
nuclei during stretching, thereby forming craze-like pores holding
fine particles in the center. As a result, the battery separator
formed by the microporous polyethylene membrane has improved
compression resistance and electrolytic solution absorbability. The
sizes of the spherical fine particles and the major axes of the
ellipsoidal fine particles are preferably 0.1 to 15 .mu.m, more
preferably 0.5 to 10 .mu.m, particularly 1 to 10 .mu.m.
[0028] When the crystalline resin having a melting point of lower
than 150.degree. C. or the amorphous resin having Tg of lower than
150.degree. C. is used, the resin is highly dispersed in the
homopolymer or composition described in (a)-(d) above during melt
blending, failing to form fine particles having proper diameters.
As a result, small gaps are formed by cleavage with fine resin
particles as nuclei, failing to expect further improvement in
compression resistance and electrolytic solution absorbability. The
upper limit of the melting point or Tg of the heat-resistant resin
is preferably 350.degree. C. from the aspect of blendability with
the homopolymer or composition described in (a)-(d) above, through
not particularly critical. The melting point or Tg of the
heat-resistant resin is more preferably 170 to 260.degree. C.
[0029] The Mw of the heat-resistant resin is preferably
1.times.10.sup.3 to 1.times.10.sup.6, more preferably
1.times.10.sup.4 to 7.times.10.sup.5, though variable depending on
the type of the resin. The heat-resistant resin having Mw of less
than 1.times.10.sup.3 is highly dispersed in the homopolymer or
composition described in (a)-(d) above, failing to form fine
particles. The heat-resistant resin having Mw of more than
1.times.10.sup.6 cannot easily be blended with the homopolymer or
composition described in (a)-(d) above.
[0030] The heat-resistant resin content is preferably 3 to 30% by
mass, more preferably 5 to 25% by mass, based on 100% by mass of
the entire heat-resistant polyethylene resin composition. When this
content is more than 30% by mass, the membrane has low pin puncture
strength, tensile rupture strength and flatness.
[0031] Specific examples of the heat-resistant resin include
polyesters, polypropylene (PP), polymethylpentene [PMP or TPX
(transparent polymer X)], fluororesins, polyamides (PA, melting
point: 215 to 265.degree. C.), polyarylene sulfides (PAS),
polystyrene (PS, melting point: 230.degree. C.), polyvinyl alcohol
(PVA, melting point: 220 to 240.degree. C.), polyimides (PI, Tg:
280.degree. C. or higher), polyamideimides (PAI, Tg: 280.degree.
C.), polyethersulfone (PES, Tg: 223.degree. C.),
polyetheretherketone (PEEK, melting point: 334.degree. C.),
polycarbonates (PC, melting point: 220 to 240.degree. C.),
cellulose acetate (melting point: 220.degree. C.), cellulose
triacetate (melting point: 300.degree. C.), polysulfone (Tg:
190.degree. C.), polyetherimides (melting point: 216.degree. C.),
etc. The heat-resistant resin can be composed of not only a single
resin component but also pluralities of resin components.
[0032] (1) Polyesters
[0033] The polyesters include polybutylene terephthalate (PBT,
melting point: about 160 to 230.degree. C.), polyethylene
terephthalate (PET, melting point: about 250 to 270.degree. C.),
polyethylene naphthalate (PEN, melting point: 272.degree. C.),
polybutylene naphthalate (PBN, melting point: 245.degree. C.),
etc., and PBT is preferable.
[0034] The PBT is essentially a saturated polyester composed of
1,4-butanediol and terephthalic acid. Within ranges not
deteriorating properties such as heat resistance, compression
resistance, heat shrinkage resistance, etc., other diols than
1,4-butanediol and other carboxylic acids than terephthalic acid
can be included as comonomers. Such diols can be, for instance,
ethylene glycol, diethylene glycol, neopentyl glycol,
1,4-cyclohexanemethanol, etc. The dicarboxylic acids can be, for
instance, isophthalic acid, sebacic acid, adipic acid, azelaic
acid, succinic acid, etc. A specific example of PBT resin forming
the PBT can be, for instance, a homo-PBT resin commercially
available from Toray Industries, Inc. under the tradename of
"Toraycon." The PBT can be composed of not only a single component
but also pluralities of PBT resin components. The PBT particularly
has Mw of 2.times.10.sup.4 to 3.times.10.sup.5.
[0035] (2) Polypropylene
[0036] PP can be not only a homopolymer, but also a block or random
copolymer containing other .alpha.-olefins or diolefins. The other
olefins are preferably ethylene or .alpha.-olefins having 4 to 8
carbon atoms. The .alpha.-olefins having 4 to 8 carbon atoms
include, for instance, 1-butene, 1-hexene, 4-methyl-1-pentene, etc.
The diolefins preferably have 4 to 14 carbon atoms. The diolefins
having 4 to 14 carbon atoms include, for instance, butadiene,
1,5-hexadiene, 1,7-octadiene, 1,9-decadiene, etc. The content of
the other olefin or diolefin is preferably less than 10% by mole,
based on 100% by mole of the propylene copolymer.
[0037] The PP particularly has Mw of 1.times.10.sup.5 to
8.times.10.sup.5. The molecular weight distribution (Mw/Mn) of the
PP is preferably 1.01 to 100, more preferably 1.1 to 50. The PP can
be a single substance or a composition of two or more types of PP.
The PP preferably has a melting point of 155 to 175.degree. C.
Because such PP is dispersed in the form of fine particles having
shapes and particle sizes as described above in the polyethylene
resin, fibrils constituting the microporous membrane are cleft with
fine PP particles as nuclei, thereby providing pores formed by
craze-like gaps.
[0038] (3) Polymethylpentene
[0039] PMP is essentially a polyolefin constituted by any one of
4-methyl-1-pentene, 2-methyl-1-pentene, 2-methyl-2-pentene,
3-methyl-l-pentene and 3-methyl-2-pentene, and a 4-methyl-1-pentene
homopolymer is preferable. PMP can be a copolymer containing a
small amount of an .alpha.-olefin other than methylpentene within a
range not deteriorating properties such as heat resistance,
compression resistance, heat shrinkage resistance, etc. The
.alpha.-olefins other than methylpentene are suitably ethylene,
propylene, butene-1, pentene-1, hexene-1, octene, vinyl acetate,
methyl methacrylate, styrene, etc. PMP usually has a melting point
of 230 to 245.degree. C. PMP particularly has Mw of
3.times.10.sup.5 to 7.times.10.sup.5.
[0040] (4) Fluororesins
[0041] The fluororesins include polyvinylidene fluoride (PVDF,
melting point: 171.degree. C.), polytetrafluoroethylene (PTFE,
melting point: 327.degree. C.), a
tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA,
melting point: 310.degree. C.), a
tetrafluoroethylene-hexafluoropropylene-perfluoro(propylvinyl
ether) copolymer (EPE, melting point: 295.degree. C.), a
tetrafluoroethylene-hexafluoropropylene copolymer (FEP, melting
point: 275.degree. C.), an ethylene-tetrafluoroethylene copolymer
(ETFE, melting point: 270.degree. C.), etc.
[0042] The fluororesin is preferably PVDF. PVDF can be a copolymer
(vinylidene fluoride copolymer) with other olefins. The vinylidene
fluoride content in the vinylidene fluoride copolymer is preferably
75% or more by mass, more preferably 90% or more by mass. Monomers
copolymerizable with vinylidene fluoride include
hexafluoropropylene, tetrafluoroethylene, trifluoropropylene,
ethylene, propylene, isobutylene, styrene, vinyl chloride,
vinylidene chloride, difluorochloroethylene, vinyl formate, vinyl
acetate, vinyl propionate, vinyl butyrate, acrylic acid and its
salts, methyl methacrylate, allyl methacrylate, acrylonitrile,
methacrylonitrile, N-butoxymethyl acrylamide, allyl acetate,
isopropenyl acetate, etc. The preferred vinylidene fluoride
copolymer is a hexafluoropropylene-vinylidene fluoride
copolymer.
[0043] (5) Polyamides
[0044] PA is preferably at least one selected from the group
consisting of polyamide 6 (6-nylon), polyamide 66 (6,6-nylon),
polyamide 12 (12-nylon) and amorphous polyamide.
[0045] (6) Polyarylene Sulfides
[0046] PAS is preferably polyphenylene sulfide (PPS) having a
melting point of 285.degree. C. PPS can be linear or branched.
[0047] (f) Molecular Weight Distribution Mw/Mn
[0048] Mw/Mn is a measure of a molecular weight distribution, the
larger this value, the wider the molecular weight distribution.
Though not critical, the Mw/Mn of the polyethylene resin is
preferably 5 to 300, more preferably 10 to 100, when the
polyethylene resin is composed of the ultra-high-molecular-weight
polyethylene, the other polyethylene, or the polyethylene
composition. When the Mw/Mn is less than 5, there are excessive
high-molecular weight components, resulting in difficulty in melt
extrusion. When the Mw/Mn is more than 300, there are excessive
low-molecular weight components, resulting in a microporous
membrane with decreased strength. The Mw/Mn of polyethylene
(homopolymer or ethylene-.alpha.-olefin copolymer) can be properly
controlled by multi-stage polymerization. The multi-stage
polymerization method is preferably a two-stage polymerization
method comprising forming a high-molecular-weight polymer component
in the first stage and forming a low-molecular-weight polymer
component in the second stage. In the case of the polyethylene
composition, the larger the Mw/Mn, the larger difference in Mw
between the ultra-high-molecular-weight polyethylene and the other
polyethylene, and vice versa. The Mw/Mn of the polyethylene
composition can be properly controlled by the molecular weight and
percentage of each component.
[0049] [2] Production Method of Microporous Polyethylene
Membrane
[0050] (a) First Production Method
[0051] The first method of this invention for producing a
microporous polyethylene membrane comprises the steps of (1) (i)
melt-blending the polyethylene resin and the membrane-forming
solvent to prepare a polyethylene resin solution A having a resin
concentration of 25 to 50% by mass, (ii) melt-blending the
polyethylene resin and the membrane-forming solvent to prepare a
polyethylene resin solution B having a resin concentration of 10 to
30% by mass, the resin concentration in the polyethylene resin
solution B being lower than that in the polyethylene resin solution
A, (2) simultaneously extruding the polyethylene resin solutions A
and B through a die, (3) cooling the resultant laminate extrudate
to provide a gel-like sheet, (4) removing the membrane-forming
solvent from the gel-like sheet, and (5) drying the resultant
membrane. Before the step (4), if necessary, a stretching step, a
heat-setting step, a heat roll treatment step and a hot solvent
treatment step can be conducted. After the step (5), a
re-stretching step, a hot solvent treatment step, a heat treatment
step, a cross-linking step with ionizing radiations, a
hydrophilizing step, a surface-coating step, etc. can be
conducted.
[0052] (1) Preparation of Polyethylene Resin Solution
[0053] (i) Preparation of Polyethylene Resin Solution A
[0054] The above polyethylene resin (called "polyethylene resin A"
unless otherwise mentioned) and a proper membrane-forming solvent
are melt-blended to prepare a polyethylene resin solution A
(hereinafter referred to simply as "resin solution A"). The resin
solution A can contain various additives such as fillers,
antioxidants, ultraviolet absorbents, antiblocking agents,
pigments, dyes, etc., if necessary, in ranges not deteriorating the
effects of this invention. Fine silicate powder, for instance, can
be added as a pore-forming agent.
[0055] The membrane-forming solvent can be liquid or solid. The
liquid solvents can be aliphatic or cyclic hydrocarbons such as
nonane, decane, decalin, p-xylene, undecane, dodecane, liquid
paraffin, etc.; and mineral oil distillates having boiling points
corresponding to those of the above hydrocarbons. To obtain a
gel-like sheet having a stable liquid solvent content, non-volatile
liquid solvents such as liquid paraffin are preferable. The solid
solvent preferably has melting point of 80.degree. C. or lower.
Such solid solvents are paraffin wax, ceryl alcohol, stearyl
alcohol, dicyclohexyl phthalate, etc. The liquid solvent and the
solid solvent can be used in combination.
[0056] The viscosity of the liquid solvent is preferably 30 to 500
cSt, more preferably 50 to 200 cSt, at a temperature of 25.degree.
C. When this viscosity is less than 30 cSt, the resin solution A is
unevenly extruded through a die lip, resulting in difficulty in
blending. The viscosity of more than 500 cSt makes the removal of
the liquid solvent difficult.
[0057] The fillers can be inorganic or organic fillers. The
inorganic fillers include silica, alumina, silica-alumina, zeolite,
mica, clay, kaolin, talc, calcium carbonate, calcium oxide, calcium
sulfate, barium carbonate, barium sulfate, magnesium carbonate,
magnesium sulfate, magnesium oxide, diatomaceous earth, glass
powder, aluminum hydroxide, titanium dioxide, zinc oxide, satin
white, acid clay, etc. The inorganic fillers can be used alone or
in combination. Among them, silica and/or calcium carbonate are
preferably used. The organic fillers are preferably made of the
above heat-resistant resins.
[0058] The shapes of filler particles are not particularly
critical, but spherical or pulverized fillers, for instance, can be
properly selected, and spherical fillers are preferable. The
particle size of the fillers is preferably 0.1 to 15 .mu.m, more
preferably 0.5 to 10 .mu.m. The fillers can be surface-treated.
Surface-treating agents for the fillers include, for instance,
various silane coupling agents, aliphatic acids such as stearic
acid or their derivatives, etc.
[0059] The use of fillers improves the electrolytic solution
absorbability. This appears to be due to the fact that with fillers
added, fibrils constituting the microporous membrane are cleft with
filler particles as nuclei, thereby forming craze-like gaps (pores)
and thus increasing the volume of gaps (pores). It is presumed that
filler particles are held in such pores.
[0060] The amount of fillers added is preferably 0.1 to 5 parts by
mass, more preferably 0.5 to 3 parts by mass, based on 100 parts by
mass of the total amount of the polyethylene resin A and the
fillers. When this content is more than 5 parts by mass, the
membrane has low pin puncture strength and deteriorates
deformability by compression, resulting in increased detachment of
fillers while slitting. A large amount of powder generated by the
detachment of fillers is likely to form defects such as pinholes,
specks (impurity), etc. in the microporous membrane products.
[0061] Though not particularly critical, uniform melt blending in a
double-screw extruder is preferable. This method is suitable for
preparing a high-concentration solution of the polyethylene resin
A. The melt-blending temperature is preferably the melting point
Tm.sub.a of the polyethylene resin A+10.degree. C. to the melting
point Tm.sub.a+100.degree. C. The melting point Tm.sub.a of the
polyethylene resin A is the melting point of (a)
ultra-high-molecular-weight polyethylene, (b) polyethylene other
than the ultra-high-molecular-weight polyethylene, or (c) a
polyethylene composition, when the polyethylene resin A is any one
of (a) to (c). When the polyethylene resin A is (d) a polyolefin
composition or (e) a heat-resistant polyethylene resin composition,
the melting point Tm.sub.a of the polyethylene resin A is the
melting point of the above (a) to (c) contained in (d) the
polyolefin composition or (e) the heat-resistant polyethylene resin
composition. The ultra-high-molecular-weight polyethylene described
in [1] (a) above, the polyethylene other than the
ultra-high-molecular-weight polyethylene described in [1] (b)
above, and the polyethylene composition described in [1] (c) above
have melting points of about 130 to 140.degree. C. Accordingly, the
melt-blending temperature is preferably in a range of 140 to
250.degree. C., more preferably in a range of 170 to 240.degree.
C.
[0062] When the polyethylene resin A is the heat-resistant
polyethylene resin composition, the melt-blending temperature is
more preferably in a range from the melting point Tm.sub.a of the
crystalline, heat-resistant resin or the Tg of the amorphous,
heat-resistant resin to the melting point Tm.sub.a+100.degree. C.,
depending on the type of the heat-resistant resin. For instance,
when the heat-resistant resin is PP having a melting point of 155
to 175.degree. C. or PBT having a melting point of about 160 to
230.degree. C., the melt-blending temperature is preferably 160 to
260.degree. C., more preferably 180 to 250.degree. C.
[0063] The membrane-forming solvent can be added before starting
the melt blending, or charged into the extruder at an intermediate
position during the melt blending, though the latter is preferable.
In the melt blending, an antioxidant is preferably added to prevent
the oxidization of the polyethylene resin A.
[0064] A ratio L/D, in which L and D respectively represent the
length and diameter of the screws in the double-screw extruder, is
preferably 20 to 100, more preferably 35 to 70. When L/D is less
than 20, enough melt blending is not achieved. When L/D is more
than 100, there is too much residing time for the resin solution A.
The screw is not particularly critical but can be of known shape. A
cylinder of the double-screw extruder preferably has an inner
diameter of 40 to 100 mm.
[0065] The resin concentration in the resin solution A is 25 to 50%
by mass, preferably 25 to 45% by mass, based on 100% by mass of the
total amount of the polyethylene resin A and the membrane-forming
solvent. When this resin concentration is less than 25% by mass,
the microporous layer A formed by the resin solution A is unlikely
to have a dense structure in the resultant microporous membrane.
When the resin concentration is more than 50% by mass, the gel-like
molding has poor formability.
[0066] (ii) Preparation of Polyethylene Resin Solution B
[0067] The polyethylene resin solution B (hereinafter referred to
simply as "resin solution B") can be the same as described above,
except that the resin concentration of the polyethylene resin
(referred to as "polyethylene resin B" unless otherwise mentioned)
is 10 to 30% by mass based on 100% by mass of the total amount of
the polyethylene resin B and the membrane-forming solvent, and
lower than that in the resin solution A. Less than 10% by mass of
the resin concentration undesirably causes decrease in
productivity. In addition, large swelling and neck-in occur at the
die exit in the extrusion of the resin solution B, resulting in
decrease in the formability and self-supportability of the gel-like
molding. More than 30% by mass of this resin concentration makes it
difficult to provide the microporous layer B produced from the
resin solution B with a coarse structure in the resultant
microporous membrane. This resin concentration is preferably 10 to
25% by mass.
[0068] The melt-blending temperature is preferably in a range from
the melting point Tm.sub.b of the polyethylene resin B+10.degree.
C. to the melting point Tm.sub.b+100.degree. C. When the
polyethylene resin B is (a) the ultra-high-molecular-weight
polyethylene, (b) the polyethylene other than the
ultra-high-molecular-weight polyethylene, or (c) the polyethylene
composition, the melting point Tm.sub.b of the polyethylene resin B
is a melting point of any one of them. When the polyethylene resin
B is (d) the polyolefin composition or (e) the heat-resistant
polyethylene resin composition, the melting point Tm.sub.b is a
melting point of any one of (a) to (c) above, which is contained in
(d) the polyolefin composition or (e) the heat-resistant
polyethylene resin composition. When the polyethylene resin B is
the heat-resistant polyethylene resin composition, the
melt-blending temperature is more preferably in a range from the
melting point Tm.sub.b of the crystalline, heat-resistant resin or
the Tg of the amorphous, heat-resistant resin to the melting point
Tm.sub.b+100.degree. C., depending on the type of the
heat-resistant resin.
[0069] (iii) Concentration Difference Between Polyethylene Resin
Solutions A and B
[0070] With the resin solution A having a higher resin
concentration than that of the resin solution B, the resultant
microporous polyethylene membrane has a gradient structure, in
which an average pore diameter in the microporous layer B is larger
than that in the microporous layer A. Accordingly, this invention
can provide a microporous polyethylene membrane with an average
pore diameter changing in a thickness direction, without stretching
the gel-like sheet. The resin concentration difference between the
resin solutions A and B is preferably 5% or more by mass, more
preferably 10% or more by mass.
[0071] (2) Extrusion
[0072] The melt-blended resin solutions A and B are supplied from
separate extruders to a die, through which they are simultaneously
extruded. In the simultaneous extrusion of the resin solutions A
and B, in which both solutions are combined in a laminar manner in
one die and extruded in a sheet form (bonding inside the die),
pluralities of extruders are connected to one die. Alternatively,
when both solutions are extruded in a sheet form from separate dies
and then laminated (bonding outside the die), each extruder is
connected to each die. Bonding inside the die is preferable.
[0073] In the simultaneous extrusion, either a flat die method or
an inflation method can be used. To achieve bonding inside the die
in either method, a method of supplying the solutions to each
manifold connected to each multi-layer-forming die and laminating
them in a laminar manner at a die lip (multi-manifold method), or a
method of laminating the solutions in a laminar manner and then
supplying the resultant laminate to a die (block method) can be
used. Because the multi-manifold method and the block method per se
are known, their detailed description will be omitted. For
instance, a known flat or inflation die can be used to form a
multi-layer membrane. The multi-layer-forming flat die preferably
has a gap of 0.1 to 5 mm. When bonding is conducted outside the die
by the flat die method, sheet-shaped solutions extruded through
each die can be laminated under pressure between a pair of rolls.
In any methods described above, the die is heated at a temperature
of 140 to 250.degree. C. during extrusion. The extrusion speed of
the heated solution is preferably 0.2 to 15 m/minute. The
adjustment of the amount of each resin solution A, B extruded can
determine a ratio of the microporous layer A to the microporous
layer B.
[0074] (3) Formation of Gel-Like Sheet
[0075] The resultant laminate extrudate is cooled to provide a
gel-like sheet. The cooling is preferably conducted to at least a
gelation temperature at a speed of 50.degree. C./minute or more.
Such cooling provides a fixed microphase separation between the
polyethylene resins A and B caused by the membrane-forming solvent.
The cooling is preferably conducted to 25.degree. C. or lower. In
general, a low cooling speed provides the gel-like sheet with a
coarse high-order structure having large pseudo-cell units, while a
high cooling speed provides dense cell units. The cooling speed of
less than 50.degree. C./minute increases crystallization, making it
difficult to form a stretchable gel-like sheet. The cooling method
can be a method of bringing the extrudate into contact with a
cooling medium such as a cooling air, a cooling water, etc., a
method of bring the extrudate into contact with a cooling roll,
etc.
[0076] When the polyethylene resins A and B are any one of [1]
(a)-(e) above, the cooling roll temperature is preferably in a
range from Tc-115.degree. C. to Tc, wherein Tc is lower one of the
crystallization temperature Tc.sub.a of the polyethylene resin A
and the crystallization temperature Tc.sub.b of the polyethylene
resin B. The cooling roll temperature exceeding the crystallization
temperature Tc fails to provide sufficiently rapid cooling. The
cooling roll temperature is more preferably in a range from the
crystallization temperature Tc-110.degree. C. to the
crystallization temperature Tc-10.degree. C. When the polyethylene
resin A, B is (a) the ultra-high-molecular-weight polyethylene, (b)
the polyethylene other than the ultra-high-molecular-weight
polyethylene, or (c) the polyethylene composition, the
crystallization temperature Tc.sub.a, Tc.sub.b of the polyethylene
resin A, B is a crystallization temperature of any one of (a)-(c).
When the polyethylene resin A, B is (d) the polyolefin composition
or (e) the heat-resistant polyethylene resin composition, the
crystallization temperature Tc.sub.a, Tc.sub.b of the polyethylene
resin A, B is a crystallization temperature of any one of (a)-(c)
above, which is contained in (d) the polyolefin composition or (e)
the heat-resistant polyethylene resin composition.
[0077] The crystallization temperature is measured according to JIS
K7121. The crystallization temperatures of the
ultra-high-molecular-weight polyethylene described in [1] (a)
above, the polyethylene other than the ultra-high-molecular-weight
polyethylene described in [1] (b) above, and the polyethylene
composition described in [1] (c) above are generally 102 to
108.degree. C. Accordingly, the cooling roll temperature is in a
range from -10.degree. C. to 105.degree. C., preferably in a range
from -5.degree. C. to 95.degree. C. The contact time between the
cooling roll and the sheet is preferably 1 to 30 seconds, more
preferably 2 to 15 seconds.
[0078] (4) Removal of Membrane-Forming Solvent
[0079] The membrane-forming solvent is removed (washed away) using
a washing solvent. Because the phases of the polyethylene resins A
and B are separated from the membrane-forming solvent phase, the
removal of the membrane-forming solvent provides a microporous
membrane composed of fibrils constituting a fine, three-dimensional
network structure and having three-dimensionally and irregularly
communicating pores (voids). The washing solvents can be volatile
solvents, for instance, saturated hydrocarbons such as pentane,
hexane, heptane, etc.; chlorinated hydrocarbons such as methylene
chloride, carbon tetrachloride, etc.; ethers such as diethyl ether,
dioxane, etc.; ketones such as methyl ethyl ketone, etc.; linear
fluorocarbons such as trifluoroethane, C.sub.6F.sub.14,
C.sub.7F.sub.16, etc.; cyclic hydrofluorocarbons such as
C.sub.5H.sub.3F.sub.7, etc.; hydrofluoroethers such as
C.sub.4F.sub.9OCH.sub.3, C.sub.4F.sub.9OC.sub.2H.sub.5, etc.; and
perfluoroethers such as C.sub.4F.sub.9OCF.sub.3,
C.sub.4F.sub.9OC.sub.2F.sub.5, etc. These washing solvents have a
low surface tension, for instance, 24 mN/m or less at 25.degree. C.
The use of a washing solvent having a low surface tension
suppresses a pore-forming network structure from shrinking due to a
surface tension of gas-liquid interfaces during drying after
washing, thereby providing a microporous membrane having high
porosity and permeability.
[0080] The washing of the gel-like sheet can be conducted by a
washing-solvent-immersing method, a washing-solvent-showering
method, or a combination thereof. The amount of the washing solvent
used is preferably 300 to 30,000 parts by mass per 100 parts by
mass of the membrane. The washing temperature can usually be 15 to
30.degree. C., and heat-washing can be conducted, if necessary. The
heat-washing temperature is preferably 80.degree. C. or lower.
Washing with the washing solvent is preferably conducted until the
amount of the remaining membrane-forming solvent becomes less than
1% by mass of that added.
[0081] (5) Drying of Membrane
[0082] The microporous polyethylene membrane obtained by removing
the membrane-forming solvent is then dried by a heat-drying method,
a wind-drying method, etc. The drying temperature is preferably
equal to or lower than a crystal dispersion temperature Tcd, which
is a lower one of the crystal dispersion temperature Tcd.sub.a of
the polyethylene resin A and the crystal dispersion temperature
Tcd.sub.b of the polyethylene resin B, particularly 5.degree. C. or
more lower than the crystal dispersion temperature Tcd. When the
polyethylene resin A, B is (a) the ultra-high-molecular-weight
polyethylene, (b) the polyethylene other than the
ultra-high-molecular-weight polyethylene, or (c) the polyethylene
composition, the crystal dispersion temperature Tcd.sub.a,
Tcd.sub.b of the polyethylene resin A, B is a crystal dispersion
temperature of (a) to (c) above. When the polyethylene resin A, B
is (d) the polyolefin composition, or (e) the heat-resistant
polyethylene resin composition, it is a crystal dispersion
temperature of (a) to (c) above, which is contained in (d) the
polyolefin composition or (e) the heat-resistant polyethylene resin
composition. The crystal dispersion temperature is determined by
measuring the temperature properties of dynamic viscoelasticity
according to ASTM D 4065. The ultra-high-molecular-weight
polyethylene in [1] (a) above, the polyethylene other than the
ultra-high-molecular-weight polyethylene in [1] (b) above, and the
polyethylene composition in [1] (c) have crystal dispersion
temperatures in a range of above 90 to 100.degree. C.
[0083] Drying is conducted until the percentage of the remaining
washing solvent becomes preferably 5% or less by mass, more
preferably 3% or less by mass, based on 100% by mass of the
microporous membrane (dry weight). Insufficient drying undesirably
reduces the porosity of the microporous membrane in subsequent
re-stretching and heat treatment steps, thereby resulting in poor
permeability.
[0084] (6) Optional Steps Before Removal of Membrane-Forming
Solvent
[0085] Before the step (4) of removing the membrane-forming
solvent, any one of a stretching step, a heat-setting step, a heat
roll treatment step and a hot solvent treatment step can be
conducted.
[0086] (i) Stretching
[0087] After heating, the gel-like sheet is preferably stretched to
a predetermined magnification by a tenter method, a roll method, an
inflation method, a rolling method, or their combination. Because
the gel-like sheet contains a membrane-forming solvent, it can be
uniformly stretched. Although the stretching can be monoaxial or
biaxial, biaxial stretching is preferable. The biaxial stretching
can be simultaneous biaxial stretching, sequential stretching, or
multi-stage stretching (for instance, a combination of simultaneous
biaxial stretching and sequential stretching), though the
simultaneous biaxial stretching is particularly preferable.
[0088] The stretching magnification is preferably 2-fold or more,
more preferably 3- to 30-fold in the case of monoaxial stretching.
In the case of biaxial stretching, it is at least 3-fold in both
directions, with an area magnification of preferably 9-fold or
more, more preferably 25-fold or more. The area magnification of
less than 9-fold results in insufficient stretching, failing to
providing a high-modulus, high-strength microporous membrane. When
the area magnification is more than 400-fold, restrictions occur on
stretching apparatuses, stretching operations, etc.
[0089] The stretching temperature is preferably equal to or lower
than a melting point Tm+10.degree. C., more preferably in a range
of the crystal dispersion temperature Tcd or more and lower than
the melting point Tm, the melting point Tm being lower one of the
melting point Tm.sub.a of the polyethylene resin A and the melting
point Tm.sub.b of the polyethylene resin B. When this stretching
temperature exceeds the melting point Tm+10.degree. C., the resin
is molten, so that stretching fails to orient molecular chains.
When it is lower than the crystal dispersion temperature Tcd, the
resin is insufficiently softened, making it likely that the
membrane is broken by stretching, thus failing to achieve
high-magnification stretching. As described above, the
ultra-high-molecular-weight polyethylene described in [1] (a)
above, the polyethylene other than the ultra-high-molecular-weight
polyethylene described in [1] (b) above, and the polyethylene
composition described in [1] (c) above have crystal dispersion
temperatures of about 90 to 100.degree. C. Accordingly, the
stretching temperature is usually in a range of 90 to 140.degree.
C., preferably in a range of 100 to 130.degree. C.
[0090] The above stretching causes cleavage between polyethylene
crystal lamellas, making the polyethylene phase
(ultra-high-molecular-weight polyethylene phase, polyethylene phase
or polyethylene composition phase) finer with larger numbers of
fibrils. The fibrils form a three-dimensional network structure
(three-dimensionally and irregularly connected network structure).
In a layer containing the heat-resistant polyethylene resin
composition, fibrils are cleft with fine, heat-resistant resin
particles as nuclei, thereby forming craze-like pores holding fine
particles.
[0091] Depending on the desired properties, stretching can be
conducted with a temperature distribution in a thickness direction,
to provide a microporous membrane with higher mechanical strength.
This method is described specifically in Japanese Patent
3347854.
[0092] (ii) Heat-Setting
[0093] The gel-like sheet can be heat-set. The heat-setting can
change the pore size and porosity of the microporous membrane, and
particularly enlarge the pore size of the microporous layer B. The
heat-setting is conducted by a tenter method, a roll method or a
rolling method. The heat-setting is conducted in a temperature
range of the melting point Tm+10.degree. C. or lower, preferably
from the crystal dispersion temperature Tcd to the melting point
Tm.
[0094] (iii) Hot Roll Treatment
[0095] At least one surface of the gel-like sheet can be brought
into contact with a heat roll (heat roll treatment), to enlarge the
pore diameter near the surface. The pore diameter near the surface
and the thickness of a layer having enlarged pore diameter can be
controlled by adjusting the roll temperature, the contact time of
the membrane with the roll, the contact area ratio of the membrane
with the roll, etc.
[0096] The roll temperature is preferably in a range of the crystal
dispersion temperature Tcd+10.degree. C. or higher and lower than
the melting point Tm. The heat roll treatment is preferably
conducted on the stretched gel-like sheet. The heat-stretched
gel-like sheet is preferably cooled to a temperature lower than the
crystal dispersion temperature Tcd before contact with the heat
roll.
[0097] The roll can have a smooth or rough surface. A smooth roll
can be a rubber or metal roll. The heat roll can have a function of
sucking the gel-like sheet. When the gel-like sheet comes into
contact with the heat roll having a heating oil on the surface,
high heating efficiency is achieved, and the resultant membrane is
provided with a larger average pore diameter near the surface. The
heating oil can be the same as the membrane-forming solvent. The
use of a suction roll can control the amount of the
membrane-forming solvent kept on the roll.
[0098] (iv) Hot Solvent Treatment
[0099] The gel-like sheet can be treated with a hot solvent. The
hot solvent treatment is preferably conducted on the stretched
gel-like sheet. Solvents usable for the heat treatment are
preferably the above liquid membrane-forming solvents, more
preferably liquid paraffin. The heat treatment solvents can be the
same as or different from the membrane-forming solvent used for
producing the resin solution A or B.
[0100] The hot solvent treatment method is not particularly
critical as long as the gel-like sheet comes into contact with a
hot solvent. It includes, for instance, a method of directly
contacting the gel-like sheet with a hot solvent (simply called
"direct method" unless otherwise mentioned), a method of contacting
the gel-like sheet with a cold solvent and then heating it (simply
called, "indirect method" unless otherwise mentioned), etc. The
direct method includes a method of immersing the gel-like sheet in
a hot solvent, a method of spraying a hot solvent to the gel-like
sheet, a method of coating the gel-like sheet with a hot solvent,
etc., and the immersing method is preferable. In the indirect
method, the gel-like sheet is immersed in a cold solvent, sprayed
with a cold solvent, or coated with a cold solvent, and then
brought into contact with a heat roll, heated in an oven, or
immersed in a hot solvent.
[0101] With the temperature and time properly set in the hot
solvent treatment, the pore size and porosity of the microporous
membrane can be changed. Particularly the pore size in the
coarse-structure layer (microporous layer B) can be increased. The
hot solvent temperature is preferably in a range from the crystal
dispersion temperature Tcd to the melting point Tm+10.degree. C.
Specifically, the hot solvent temperature is preferably 110 to
140.degree. C., more preferably 115 to 135.degree. C. The contact
time is preferably 0.1 seconds to 10 minutes, more preferably 1
second to 1 minute. When the hot solvent temperature is lower than
the crystal dispersion temperature Tcd, or when the contact time is
less than 0.1 seconds, the hot solvent treatment has substantially
no effect, only with little improvement in the permeability. When
the hot solvent temperature is higher than the melting point
Tm+10.degree. C., or when the contact time is longer than 10
minutes, the microporous membrane is undesirably provided with
decreased strength or broken.
[0102] With such hot solvent treatment, fibrils formed by
stretching have a leaf-vein-like structure, in which trunk-forming
fibers are relatively thick. Accordingly, the microporous membrane
having a large pore size and excellent strength and permeability
can be obtained. The term "leaf-vein-like fibrils" means that the
fibrils have thick trunks and fine fibers spreading from the
trunks, forming a complex network structure.
[0103] Although the remaining heat treatment solvent is removed by
washing after the hot solvent treatment, it can be removed together
with the membrane-forming solvent.
[0104] (7) Optional Steps After Drying Step
[0105] After the drying step (5), a re-stretching step, a heat
treatment step, a hot solvent treatment step, a cross-linking step
with ionizing radiations, a hydrophilizing step, a surface-coating
step, etc. can be conducted.
[0106] (i) Re-Stretching
[0107] A microporous membrane obtained by washing and drying the
stretched gel-like sheet is preferably stretched again in at least
one direction. The re-stretching can be conducted by the same
tenter method as described above, etc. while heating the membrane.
The re-stretching can be monoaxial or biaxial. The biaxial
stretching can be simultaneous biaxial stretching or sequential
stretching, though the simultaneous biaxial stretching is
preferable.
[0108] The re-stretching temperature is preferably the melting
point Tm or lower, more preferably in a range from the crystal
dispersion temperature Tcd to the melting point Tm. When the
re-stretching temperature exceeds the melting point Tm, the
compression resistance is deteriorated, and there is large
unevenness in properties (particularly air permeability) in a width
direction when stretched in a transverse direction (TD). When the
re-stretching temperature is lower than the crystal dispersion
temperature Tcd, the polyethylene resins A and B are insufficiently
softened, making it likely that the membrane is broken by
stretching, thus failing to achieve uniform stretching.
Specifically, the re-stretching temperature is usually in a range
of 90 to 135.degree. C., preferably in a range of 95 to 130.degree.
C.
[0109] The re-stretching magnification in one direction is
preferably 1.1- to 2.5-fold, to provide the microporous membrane
with increased pore diameter and improved compression resistance.
In the case of monoaxial stretching, for instance, it is 1.1- to
2.5-fold in MD or TD. In the case of biaxial stretching, it is 1.1-
to 2.5-fold in both MD and TD. As long as the stretching
magnification is 1.1- to 2.5-fold in each of MD and TD in biaxial
stretching, the stretching magnifications in MD and TD can be
different, but are preferably the same. When this magnification is
less than 1.1-fold, sufficiently improved compression resistance
cannot be obtained. When this magnification is more than 2.5-fold,
the membrane is highly likely broken, and undesirably suffers
decreased heat shrinkage resistance. The re-stretching
magnification is more preferably 1.1- to 2.0-fold.
[0110] (ii) Heat treatment
[0111] The dried membrane is preferably heat-treated. The heat
treatment stabilizes crystals and makes lamellas uniform. The heat
treatment can be heat setting and/or annealing. The heat-setting
treatment can be the same as described above.
[0112] The annealing can be conducted using a belt conveyer or an
air-floating furnace in addition to the tenter method, the roll
method or the rolling method. The annealing is conducted at a
temperature equal to or lower than the melting point Tm, preferably
at a temperature in a range from 60.degree. C. to the melting point
Tm-10.degree. C. Such annealing provides a high-strength,
microporous membrane with good permeability. The heat-setting and
the annealing can be combined.
[0113] (iii) Hot Solvent Treatment
[0114] The dried membrane can be treated with a hot solvent. The
hot solvent treatment can be the same as described above.
[0115] (iv) Cross-Linking of Membrane
[0116] The dried microporous membrane can be cross-linked by
ionizing radiation of .alpha.-rays, .beta.-rays, .gamma.-rays,
electron beams, etc. The electron beam irradiation is preferably
conducted at 0.1 to 100 Mrad and accelerating voltage of 100 to 300
kV. The cross-linking treatment elevates the meltdown temperature
of the multi-layer, microporous polyethylene membrane.
[0117] (v) Hydrophilizing
[0118] The dried microporous membrane can be hydrophilized. The
hydrophilizing treatment can be a monomer-grafting treatment, a
surfactant treatment, a corona-discharging treatment, etc. The
monomer-grafting treatment is preferably conducted after
cross-linking.
[0119] In case of the surfactant treatment, any of nonionic
surfactants, cationic surfactants, anionic surfactants and
amphoteric surfactants can be used, but the nonionic surfactants
are preferable. The microporous membrane is dipped in a solution of
the surfactant in water or a lower alcohol such as methanol,
ethanol, isopropyl alcohol, etc., or coated with the solution by a
doctor blade method.
[0120] (vi) Surface-Coating
[0121] The dried microporous membrane can be coated with porous
polypropylene, a porous fluororesin such as polyvinylidene fluoride
and polytetrafluoroethylene, porous polyimide, porous polyphenylene
sulfide, etc., to improve meltdown properties when used as a
battery separator. Polypropylene for a coating layer preferably has
Mw of 5,000 to 500,000, and solubility of 0.5 g or more in 100 g of
toluene at a temperature of 25.degree. C. This polypropylene more
preferably has a racemic diad fraction of 0.12 to 0.88. In the
racemic diad, two connected monomer units are in an enantiomer
relation. The coating layer can be formed, for instance, by coating
the microporous membrane with a mixed solution containing the above
coating resin and its good solvent, removing the good solvent to
increase the concentration of the resin, thereby forming a
structure in which a resin phase is separated from a good solvent
phase, and removing the remaining good solvent.
[0122] (b) Second Production Method
[0123] The second production method comprises the steps of (1)
preparing the above resin solutions A and B such that the resin
solution A has a higher concentration than that of the resin
solution B, (2) extruding the resin solutions A and B through
separate dies, (3) cooling the resultant extrudates to provide
gel-like sheets A and B, (4) removing the membrane-forming solvent
from the gel-like sheets A and B, (5) drying the resultant
microporous polyethylene membranes A and B, and (6) alternately
laminating them. Before the step (4) of removing the
membrane-forming solvent, if necessary, a step of stretching the
gel-like sheets A and B, a heat-setting step, a heat roll treatment
step and a hot solvent treatment step can be conducted. Further,
after the laminating step (6), a re-stretching step, a heat
treatment step, a hot solvent treatment step, a cross-linking step,
a hydrophilizing step, a surface-coating step, etc. can be
conducted.
[0124] Among the above steps, the step (1) can be the same as in
the first method, the step (2) can be the same as in the first
method except for extruding the resin solutions A and B through
separate dies, the step (3) can be the same as in the first method
except for forming separate gel-like sheets A and B, the step (4)
can be the same as in the first method except for removing the
membrane-forming solvent from separate gel-like sheets A and B, and
the step (5) can be the same as in the first method except for
drying separate microporous polyethylene membranes A and B. It
should be noted that in the step (5), the drying temperatures of
the microporous membranes A and B are preferably equal to or lower
than the crystal dispersion temperatures Tcd.sub.a and Tcd.sub.b,
respectively. The drying temperatures are more preferably lower
than the crystal dispersion temperatures Tcd.sub.a and Tcd.sub.b by
5.degree. C. or more.
[0125] The stretching step, the heat-setting step, the heat roll
treatment step and the hot solvent treatment step before the step
(4) can be the same as in the first method except that they are
conducted on the gel-like sheet A or B. However, when the gel-like
sheet A is stretched before the step (4), the stretching
temperature is preferably in a range of the melting point
Tm.sub.a+10.degree. C. or lower, more preferably in a range of the
crystal dispersion temperature Tcd.sub.a or higher and lower than
the melting point Tm.sub.a. When the gel-like sheet B is stretched,
the stretching temperature is preferably in a range of the melting
point Tm.sub.b+10.degree. C. or lower, more preferably in a range
of the crystal dispersion temperature Tcd.sub.b or higher and lower
than the melting point Tm.sub.b.
[0126] When the gel-like sheet A is heat-set before the step (4),
the heat-setting temperature is preferably in a range of the
melting point Tm.sub.a+10.degree. C. or lower, more preferably in a
range from the crystal dispersion temperature Tcd.sub.a to the
melting point Tm.sub.a. When the gel-like sheet B is heat-set, the
heat-setting temperature is preferably in a range of the melting
point Tm.sub.b+10.degree. C. or lower, more preferably in a range
from the crystal dispersion temperature Tcd.sub.b to the melting
point Tm.sub.b.
[0127] When the gel-like sheet A is subjected to a heat roll
treatment before the step (4), the roll temperature is preferably
in a range of the crystal dispersion temperature
Tcd.sub.a+10.degree. C. or higher and lower than the melting point
Tm.sub.a. When the gel-like sheet B is treated, the roll
temperature is more preferably in a range of the crystal dispersion
temperature Tcd.sub.b+10.degree. C. or higher and lower than the
melting point Tm.sub.b.
[0128] When the gel-like sheet A is subjected to a hot solvent
treatment before the step (4), the hot solvent temperature is
preferably in a range from the crystal dispersion temperature
Tcd.sub.a to the melting point Tm.sub.a+10.degree. C. When the
gel-like sheet B is treated, the hot solvent temperature is
preferably in a range from the crystal dispersion temperature
Tcd.sub.b to the melting point Tm.sub.b+10.degree. C.
[0129] The step (6) of alternately laminating the microporous
polyethylene membranes A and B will be described below. Though not
particularly critical, the laminating method is preferably a
heat-laminating method. The heat-laminating method includes a
heat-sealing method, an impulse-sealing method, an ultrasonic
laminating method, etc., and the heat-sealing method is preferable.
The heat-sealing method preferably uses a heat roll. In the heat
roll method, the first and second microporous polyethylene
membranes that are overlapped are heat-sealed by passing through a
pair of heat rolls or between a heat roll and a table. The
heat-sealing temperature and pressure are not particularly
critical, as long as the microporous polyethylene membranes are
sufficiently bonded, and unless the resultant microporous membrane
has poor properties. The heat-sealing temperature is, for instance,
90 to 135.degree. C., preferably 90 to 115.degree. C. The
heat-sealing pressure is preferably 0.01 to 50 MPa.
[0130] The re-stretching step, the heat treatment step, the hot
solvent treatment step, the cross-linking step, the hydrophilizing
step and the surface-coating step after the step (6) can be the
same as in the first method.
[0131] [3] Structure and Properties of Microporous Polyethylene
Membrane
[0132] The microporous polyethylene membrane produced by the method
of this invention has a gradient structure in which the microporous
layer B formed by the resin solution B has an larger average pore
diameter than that of the microporous layer A formed by the resin
solution A, so that the average pore diameter changes in a
thickness direction. The average pore diameter of the microporous
layer B is preferably 1.1-fold or more of that of the microporous
layer A.
[0133] The microporous polyethylene membrane produced by the method
of this invention comprises a microporous layer B that undergoes
large deformation when compressed and has small permeability
variation, and a microporous layer A that undergoes small
deformation when compressed. Accordingly, when the microporous
polyethylene membrane is used as a battery separator, the
microporous layer B follows the expansion and shrinkage of
electrodes while keeping permeability, and the microporous layer A
prevents short-circuiting between the electrodes.
[0134] Although the microporous polyethylene membrane usually has a
laminar structure, it can substantially be a single-layer membrane
in which the microporous layers A and B are fused in their
interface, as long as the average pore diameter changes in the
thickness direction. The number of layers in the microporous
polyethylene membrane is not particularly critical. The arrangement
of the microporous layer A and the microporous layer B is not
particularly critical, as long as the layers A and B are alternate.
In the case of a three-layer microporous membrane, for instance,
the layer arrangement can be A/B/A or B/A/B.
[0135] The thickness ratio of the microporous layer A to the
microporous layer B is not particularly critical, but can be
properly selected depending on the applications of the microporous
membrane. Adjusting the thickness ratio of the microporous layers A
and B can control balance between the compression resistance and
the electrolytic solution absorbability. When used as a battery
separator, the cross section area ratio of the microporous layer B
to the microporous layer A is preferably 0.1 to 2.5. When this
ratio is less than 0.1, the microporous membrane undergoes large
air permeability change when compressed, while having poor
electrolytic solution absorbability. When it is more than 2.5, the
microporous membrane has low mechanical strength.
[0136] When used as a liquid filter, the microporous layer A acts
as a support layer, and the microporous layer B acts as a filtering
layer. Adjusting the thickness ratio of the microporous layers A
and B can control balance between the filtering properties and the
permeability. This invention provides filters having well-balanced
filtering properties and permeability even if they are made thinner
than conventional ones.
[0137] The shape of penetrating pores is not particularly critical.
For instance, a two-layer, microporous membrane having a layer
structure of A/B can have tapered penetrating pores having large
openings on one surface, and their sizes are gradually decreasing
toward the opposite surface. A three-layer microporous membrane
having a layer structure of B/A/B, for instance, can have
penetrating pores whose sizes are gradually decreasing from both
surfaces toward the center of the membrane.
[0138] The microporous polyethylene membrane according to a
preferred embodiment of this invention has the following
properties.
[0139] (a) Porosity of 25 to 80%
[0140] With the porosity of less than 25%, the microporous
polyethylene membrane does not have good air permeability. When the
porosity exceeds 80%, the microporous polyethylene membrane used as
a battery separator does not have enough strength, resulting in a
high likelihood of short-circuiting between electrodes.
[0141] (b) Air Permeability of 20 to 500 Seconds/100 cm.sup.3
(converted to Value at 20-.mu.m Thickness)
[0142] When the air permeability is in a range from 20 to 500
seconds/100 cm.sup.3, batteries having separators formed by the
microporous polyethylene membrane have large capacity and good
cycle properties. When the air permeability exceeds 500 seconds/100
cm.sup.3, the batteries have small capacity. On the other hand,
when the air permeability is less than 20 seconds/100 cm.sup.3,
shutdown does not fully occur when the temperature is elevated in
the batteries.
[0143] (c) Pin Puncture Strength of 1,000 mN/20 .mu.m or More
[0144] With the pin puncture strength of less than 1,000 mN/20
.mu.m, a battery comprising the microporous polyethylene membrane
as a separator likely suffers short-circuiting between electrodes.
The pin puncture strength is preferably 2,000 mN/20 .mu.m or
more.
[0145] (d) Tensile Rupture Strength of 70,000 kPa or More
[0146] With the tensile rupture strength of 70,000 kPa or more in
both longitudinal direction (MD) and transverse direction (TD), the
membrane is unlikely ruptured when used as a battery separator.
[0147] (e) Tensile Rupture Elongation of 100% or More
[0148] With the tensile rupture elongation of 100% or more in both
longitudinal direction (MD) and transverse direction (TD), the
membrane is unlikely ruptured when used as a battery separator.
[0149] (f) Heat Shrinkage Ratio of 30% or Less
[0150] The heat shrinkage ratio is 30% or less in both longitudinal
direction (MD) and transverse direction (TD) after exposed to
105.degree. C. for 8 hours. When used as a battery separator, this
heat shrinkage ratio is preferably 15% or less, more preferably 10%
or less.
[0151] (g) Thickness Change Ratio of 10% or More by Heat
Compression
[0152] The thickness change ratio by heat compression at a
temperature of 90.degree. C. and a pressure of 2.2 MPa (22
kgf/cm.sup.2) for 5 minutes is 10% or more, based on 100% of the
thickness before compression. With the thickness change ratio of
10% or more, a battery separator formed by the microporous membrane
can well absorb the expansion of electrodes. This thickness change
ratio is preferably 12% or more.
[0153] (h) Post-Compression Air Permeability of 1,000 Sec/100
cm.sup.3 or Less
[0154] The post-compression air permeability (Gurley value)
measured after heat compression under the above conditions is 1,000
sec/100 cm.sup.3 or less. With the post-compression air
permeability of 1,000 sec/100 cm.sup.3 or less, a separator formed
by the microporous membrane provides a battery with large capacity
and good cycle properties. The post-compression air permeability is
preferably 900 sec/100 cm.sup.3 or less.
[0155] (i) Shutdown Temperature of 140.degree. C. or Lower
[0156] When the shutdown temperature exceeds 140.degree. C., a
lithium battery separator formed by the microporous membrane has
low shutdown response when overheated.
[0157] (j) Meltdown Temperature of 160.degree. C. or Higher
[0158] The meltdown temperature is preferably 165.degree. C. or
higher.
[0159] [4] Battery Separator
[0160] The microporous polyethylene membrane formed by the above
method has excellent mechanical properties, heat shrinkage
resistance and thermal properties with small air permeability
change when compressed, suitable for battery separators.
Particularly the microporous membrane obtained by the second method
has excellent heat shrinkage resistance. Though properly selectable
depending on the types of batteries, the thickness of the battery
separator is preferably 5 to 50 .mu.m, more preferably 10 to 35
.mu.m.
[0161] [5] Battery
[0162] The microporous polyethylene membrane of this invention can
be used preferably as a separator for secondary batteries such as
lithium secondary batteries, lithium polymer secondary batteries,
nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc
batteries, silver-zinc batteries, etc., particularly as a separator
for lithium secondary batteries. Taking the lithium secondary
battery for example, description will be made below.
[0163] The lithium secondary battery comprises a cathode and an
anode laminated via a separator, the separator containing an
electrolytic solution (electrolyte). The electrode can be of any
known structure, not particularly critical. The electrode structure
can be, for instance, a coin type in which disc-shaped cathode and
anode are opposing, a laminate type in which planar cathode and
anode are alternately laminated, a toroidal type in which
ribbon-shaped cathode and anode are wound, etc.
[0164] The cathode usually comprises (a) a current collector, and
(b) a cathodic active material layer capable of absorbing and
discharging lithium ions, which is formed on the current collector.
The cathodic active materials can be inorganic compounds such as
transition metal oxides, composite oxides of lithium and transition
metals (lithium composite oxides), transition metal sulfides, etc.
The transition metals can be V, Mn, Fe, Co, Ni, etc. Preferred
examples of the lithium composite oxides are lithium nickelate,
lithium cobaltate, lithium manganate, laminar lithium composite
oxides having an .alpha.-NaFeO.sub.2 structure, etc. The anode
comprises (a) a current collector, and (b) an anodic active
material layer formed on the current collector. The anodic active
materials can be carbonaceous materials such as natural graphite,
artificial graphite, cokes, carbon black, etc.
[0165] The electrolytic solutions can be obtained by dissolving
lithium salts in organic solvents. The lithium salts can be
LiClO.sub.4, LiPF.sub.6, LiAsF.sub.6, LiSbF.sub.6, LiBF.sub.4,
LiCF.sub.3SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2,
LiC(CF.sub.3SO.sub.2).sub.3, Li.sub.2B.sub.10Cl.sub.10,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, LiPF.sub.4(CF.sub.3).sub.2,
LiPF.sub.3(C.sub.2F.sub.5).sub.3, lower aliphatic carboxylates of
lithium, LiAlCl.sub.4, etc. The lithium salts can be used alone or
in combination. The organic solvents can be organic solvents having
high boiling points and high dielectric constants such as ethylene
carbonate, propylene carbonate, ethylmethyl carbonate,
.gamma.-butyrolactone, etc.; organic solvents having low boiling
points and low viscosity such as tetrahydrofuran,
2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl
carbonate, diethyl carbonate, etc. These organic solvents can be
used alone or in combination. Because organic solvents having high
dielectric constants have high viscosity, while those having low
viscosity have low dielectric constants, their mixtures are
preferably used.
[0166] When the battery is assembled, the separator can be
impregnated with the electrolytic solution, so that the separator
(microporous polyethylene membrane) is provided with ion
permeability. The impregnation treatment can be (and usually is)
conducted by immersing the microporous membrane in the electrolytic
solution at room temperature. When a cylindrical battery is
assembled, for instance, a cathode sheet, a separator formed by the
microporous membrane, and an anode sheet are laminated in this
order, and the resultant laminate is wound to a toroidal-type
electrode assembly. The resulting electrode assembly can be charged
into a battery can and impregnated with the above electrolytic
solution. The resulting electrode assembly can be charged into a
battery can and impregnated with the above electrolytic solution. A
battery lid acting as a cathode terminal equipped with a safety
valve can be caulked to the battery can via a gasket to produce a
battery.
[0167] This invention will be described in more detail with
reference to Examples below without intention of restricting the
scope of this invention.
EXAMPLE 1
[0168] The resin compositions A and B shown in Table 1 were
prepared to produce a microporous polyethylene membrane.
[0169] (1) Preparation of Resin Solution A
[0170] Dry-blended were 100 parts by mass of a polyethylene (PE)
composition comprising 18% by mass of ultra-high-molecular-weight
polyethylene (UHMWPE) having a mass-average molecular weight (Mw)
of 2.0.times.10.sup.6, and 82% by mass of high-density polyethylene
(HDPE) having Mw of 3.5.times.10.sup.5, with 0.2 parts by mass of
tetrakis
[methylene-3-(3,5-ditertiary-butyl-4-hydroxyphenyl)-propionate]
methane as an antioxidant. Measurement revealed that the
polyethylene composition comprising UHMWPE and HDPE had a melting
point of 135.degree. C., a crystal dispersion temperature of
100.degree. C., Mw of 6.4.times.10.sup.5, and Mw/Mn of 21.0.
[0171] The Mws of the UHMWPE, the HDPE and the PE composition were
measured by gel permeation chromatography (GPC) under the following
conditions. [0172] Measurement apparatus: GPC-150C available from
Waters Corporation, [0173] Column: Shodex UT806M available from
Showa Denko K.K., [0174] Column temperature: 135.degree. C., [0175]
Solvent (mobile phase): o-dichlorobenzene, [0176] Solvent flow
rate: 1.0 ml/minute, [0177] Sample concentration: 0.1% by mass
(dissolved at 135.degree. C. for 1 hour), [0178] Injected amount:
500 .mu.l, [0179] Detector: Differential Refractometer available
from Waters Corp., and [0180] Calibration curve: Produced from a
calibration curve of a single-dispersion, standard polystyrene
sample using a predetermined conversion constant.
[0181] 40 parts by mass of the resultant mixture was charged into a
strong-blending, double-screw extruder having an inner diameter of
58 mm and L/D of 42, and 60 parts by mass of liquid paraffin [35
cst (40.degree. C.)] was supplied to the double-screw extruder via
its side feeder. Melt-blending was conducted at 230.degree. C. and
250 rpm to prepare a resin solution A.
[0182] (2) Preparation of Resin Solution B
[0183] A resin solution B was prepared in the same manner as above
except for changing the polyethylene composition concentration to
20% by mass.
[0184] (3) Formation of Membrane
[0185] The resin solutions A and B were supplied from separate
double-screw extruders to a three-layer-forming T-die, and extruded
through the T-die such that the solution B, the solution A and the
solution B were laminated at a layer thickness ratio B/A/B of
1/1/1. The extrudate was cooled by drawing by a cooling roll
controlled at 0.degree. C, thereby obtaining a three-layer,
gel-like sheet. Using a tenter-stretching machine, the three-layer,
gel-like sheet was simultaneously and biaxially stretched at
117.5.degree. C., such that the stretching magnification was 5-fold
in both longitudinal direction (MD) and transverse direction (TD).
Fixed to an aluminum frame plate of 20 cm.times.20 cm, the
stretched three-layer, gel-like sheet was immersed in a washing
bath of methylene chloride controlled at 25.degree. C., and washed
with the vibration of 100 rpm for 3 minutes to remove the liquid
paraffin. The washed membrane was air-dried at room temperature,
and fixed to the tenter to conduct a heat-setting treatment at
128.degree. C. for 10 minutes, thereby producing a microporous
polyethylene membrane.
EXAMPLE 2
[0186] A microporous polyethylene membrane was produced in the same
manner as in Example 1, except that a three-layer, gel-like sheet
was washed and then stretched to 1.4-fold in TD at 129.degree. C.,
and that the heat-setting temperature was 129.degree. C.
EXAMPLE 3
[0187] (1) Preparation of Resin Solution A
[0188] A resin solution A having a concentration of 40% by mass was
prepared in the same manner as in Example 1 except for using a PE
composition (melting point: 135.degree. C., crystal dispersion
temperature: 100.degree. C., Mw: 9.3.times.10.sup.5, and Mw/Mn:
24.5) comprising 35% by mass of UHMWPE and 65% by mass of HDPE.
[0189] (2) Preparation of Resin Solution B
[0190] A resin solution B having a UHMWPE/HDPE mass ratio of 18/82
was prepared in the same manner as in Example 1, except that the PE
composition concentration was 15% by mass.
[0191] (3) Formation of Membrane
[0192] The resin solutions A and B were supplied from separate
double-screw extruders to a three-layer-forming T-die, and were
extruded through the T-die, such that the solution A, the solution
B and the solution A were laminated in this order at a thickness
ratio A/B/A of 1/1/1. The extrudate was cooled while drawing by a
cooling roll controlled at 0.degree. C., thereby providing a
three-layer, gel-like sheet. Using a tenter-stretching machine, the
three-layer, gel-like sheet was simultaneously and biaxially
stretched at 115.degree. C., such that the stretching magnification
was 5-fold in both longitudinal direction (MD) and transverse
direction (TD). The stretched, three-layer, gel-like sheet was
washed and air-dried in the same manner as in Example 1. The dried
membrane was stretched to 1.4-fold in TD by a tenter at
128.5.degree. C., and at 128.5.degree. C. for 10 minutes to produce
a microporous polyethylene membrane.
EXAMPLE 4
[0193] A microporous polyethylene membrane was produced in the same
manner as in Example 3, except that the layer thickness ratio of
the solution A, the solution B and the solution A in the extrudate
was 2/1/2.
EXAMPLE 5
[0194] (1) Preparation of Resin Solution A
[0195] A resin solution A having a concentration of 40% by mass was
prepared in the same manner as in Example 1, except for using a PE
composition (melting point: 135.degree. C., crystal dispersion
temperature: 100.degree. C., Mw: 4.3.times.10.sup.5, and Mw/Mn:
16.0) comprising 5% by mass of UHMWPE and 95% by mass of HDPE.
[0196] (2) Preparation of Resin Solution B
[0197] A resin solution B was prepared in the same manner as in the
above resin solution A, except that the PE composition
concentration was 20% by mass.
[0198] (3) Formation of Membrane
[0199] The resin solutions A and B were supplied from separate
double-screw extruders to a three-layer-forming T-die, and extruded
through the T-die such that the solution A, the solution B and the
solution A were laminated in this order at a layer thickness ratio
A/B/A of 1/1/1. The resultant extrudate was cooled while drawing by
a cooling roll controlled at 0.degree. C., thereby providing a
three-layer, gel-like sheet. Using a tenter-stretching machine, the
three-layer, gel-like sheet was simultaneously and biaxially
stretched at 117.5.degree. C., such that the stretching
magnification was 5-fold in both longitudinal direction (MD) and
transverse direction (TD). The stretched, three-layer, gel-like
sheet was washed and air-dried in the same manner as in Example 1.
The dried membrane was stretched to 1.4-fold in TD by a tenter at
129.degree. C., and heat-set at 129.degree. C. for 10 minutes to
produce a microporous polyethylene membrane.
EXAMPLE 6
[0200] (1) Preparation of Resin Solution A
[0201] A resin solution A having a resin concentration of 40% by
mass was prepared in the same manner as in Example 1 except for
using a composition comprising 5% by mass of UHMWPE, 90% by mass of
HDPE, and 5% by mass of PP having Mw of 5.3.times.10.sup.5, the PE
composition of UHMWPE and HDPE having melting point of 135.degree.
C., a crystal dispersion temperature of 100.degree. C., Mw of
4.4.times.10.sup.5, and Mw/Mn of 16.0.
[0202] (2) Preparation of Resin Solution B
[0203] A resin solution B was prepared in the same manner as in the
above resin solution A except for changing the resin concentration
to 20% by mass.
[0204] (3) Formation of Membrane
[0205] A microporous polyethylene membrane was produced in the same
manner as in Example 5, except for using the resultant resin
solutions A and B.
EXAMPLE 7
[0206] A microporous polyethylene membrane was produced in the same
manner as in Example 6 except for using PBT having Mw of
3.8.times.10.sup.4 in place of PP.
EXAMPLE 8
[0207] A microporous polyethylene membrane was produced in the same
manner as in Example 3, except that the simultaneously and
biaxially stretched, three-layer, gel-like sheet was heat-set at
122.degree. C. for 10 minutes and then washed, and that the
re-stretching and heat-setting were conducted at a temperature of
129.5.degree. C.
EXAMPLE 9
[0208] A microporous polyethylene membrane was produced in the same
manner as in Example 3, except that the simultaneously and
biaxially stretched, three-layer, gel-like sheet was immersed in a
liquid paraffin bath controlled at 120.degree. C. for 3 seconds and
then washed, and that the re-stretching and heat-setting were
conducted at a temperature of 130.degree. C.
EXAMPLE 10
[0209] (1) Production of Microporous Polyethylene Membrane A
[0210] A resin solution A having a resin concentration of 40% by
mass was prepared in the same manner as in Example 1, except for
using a PE composition (melting point: 135.degree. C., crystal
dispersion temperature: 100.degree. C., Mw: 4.3.times.10.sup.5, and
Mw/Mn: 16.0) comprising 5% by mass of UHMWPE and 95% by mass of
HDPE. The resin solution A was extruded from a T-die attached to a
tip end of the double-screw extruder, and cooled while drawing by a
cooling roll controlled at 0.degree. C., thereby forming a gel-like
sheet A. The gel-like sheet A was simultaneously and biaxially
stretched to 5-fold in both longitudinal direction (MD) and
transverse direction (TD) at 116.degree. C. by a tenter-stretching
machine, and then washed and air-dried in the same manner as in
Example 1 to produce a microporous polyethylene membrane A.
[0211] (2) Production of Microporous Polyethylene Membrane B
[0212] A resin solution B was prepared in the same manner as in the
above resin solution A, except that a PE composition (melting
point: 135.degree. C., crystal dispersion temperature: 100.degree.
C., Mw: 6.4.times.10.sup.5, and Mw/Mn: 21.0) comprising 18% by mass
of UHMWPE and 82% by mass of HDPE was used, and that the resin
concentration was 20% by mass. A microporous polyethylene membrane
B was produced in the same manner as in the above microporous
polyethylene membrane A except for using the resin solution B.
[0213] (3) Lamination and Heat-Setting Treatment
[0214] The microporous polyethylene membranes A and B were
laminated by passing through a pair of rolls at a temperature of
110.degree. C. and a pressure of 0.05 MPa. The resultant laminate
was heat-set at a temperature of 126.degree. C. by a tenter method
to produce a microporous polyethylene membrane, in which a layer
thickness ratio of the membrane A to the membrane B was 1/1.
EXAMPLE 11
[0215] (1) Preparation of Resin Solution A
[0216] A resin solution A was prepared in the same manner as in
Example 1, except that a PE composition (melting point: 135.degree.
C., crystal dispersion temperature: 100.degree. C., Mw:
8.2.times.10.sup.5, and Mw/Mn: 23.5) comprising 30% by mass of
UHMWPE and 70% by mass of HDPE was used, and that the resin
concentration was 30% by mass.
[0217] (2) Preparation of Resin Solution B
[0218] A resin solution B having a mass ratio UHMWPE/HDPE of 18/82
was prepared in the same manner as in Example 1, except that the
resin concentration was 15% by mass.
[0219] (3) Formation of Membrane
[0220] The resin solutions A and B were supplied from separate
double-screw extruders to a two-layer-forming T-die, and extruded
through the T-die in the form of a laminate of the solution A and
the solution B at a layer thickness ratio A/B of 1/1. The extrudate
was cooled while drawing by a cooling roll controlled at 0.degree.
C., thereby providing a two-layer, gel-like sheet. Using a
tenter-stretching machine, the two-layer, gel-like sheet was
simultaneously and biaxially stretched to 5-fold in both
longitudinal direction (MD) and transverse direction (TD) at
119.2.degree. C. The stretched, two-layer, gel-like sheet was
washed and air-dried in the same manner as in Example 1. The dried
membrane stretched to 1.4-fold by a tenter in TD at 110.degree. C.,
and heat-set at 110.degree. C. for 10 minutes to produce a
microporous polyethylene membrane.
EXAMPLE 12
[0221] Resin solutions A and B were prepared in the same manner as
in Example 11. The resin solutions A and B were extruded from
separate T-dies each attached to a tip end of each double-screw
extruder, and cooled while drawing by a cooling roll controlled at
0.degree. C., thereby providing gel-like sheets A and B. The
gel-like sheets A and B were simultaneously and biaxially stretched
to 5-fold in both longitudinal direction (MD) and transverse
direction (TD) 119.2.degree. C. by a tenter-stretching machine. The
stretched gel-like sheets A and B were washed and air-dried in the
same manner as in Example 1 to produce microporous polyethylene
membranes A and B. The microporous polyethylene membranes A and B
were laminated by passing through a pair of rolls at a temperature
of 110.degree. C. and a pressure of 0.05 MPa. The resultant
laminate was stretched to 1.4-fold in TD at a temperature of
110.degree. C. by a tenter, and heat-set at a temperature of
120.degree. C. for 10 minutes to produce a microporous polyethylene
membrane having a layer thickness ratio A/B of 1/1.
EXAMPLE 13
[0222] A resin solution A having a mass ratio UHMWPE/HDPE of 5/95
was prepared in the same manner as in Example 10 except for
changing the resin concentration to 30% by mass. A resin solution B
having a mass ratio UHMWPE/HDPE of 18/82 was prepared in the same
manner as in Example 10. The resin solutions A and B were supplied
from separate double-screw extruders to a two-layer-forming T-die,
and extruded through the T-die such that the solution A and the
solution B were laminated at a layer thickness ratio A/B of 1/1.
The resultant extrudate was cooled while drawing by a cooling roll
controlled at 90.degree. C., thereby providing a two-layer,
gel-like sheet. The two-layer, gel-like sheet was washed and
air-dried in the same manner as in Example 1, and heat-set at
125.degree. C. for 10 minutes to produce a microporous polyethylene
membrane.
EXAMPLE 14
[0223] Resin solutions A and B were prepared in the same manner as
in Example 13. The resin solution A was extruded from a T-die
attached to a tip end of a double-screw extruder, and cooled while
drawing by a cooling roll controlled at 90.degree. C., thereby
providing a gel-like sheet A. The resin solution B was extruded
from a T-die attached to a tip end of another double-screw
extruder, and cooled while drawing by a cooling roll controlled at
60.degree. C., thereby providing a gel-like sheet B. The gel-like
sheets A and B were washed and air-dried in the same manner as in
Example 1 to produce microporous polyethylene membranes A and B.
The microporous polyethylene membranes A and B were laminated by
passing through a pair of rolls at a temperature of 110.degree. C.
and at a pressure of 0.05 MPa, and heat-set at 128.degree. C. for
10 minutes to produce a microporous polyethylene membrane having a
layer thickness ratio A/B of 1/1.
COMPARATIVE EXAMPLE 1
[0224] A resin solution was prepared in the same manner as in
Example 1, except that a PE composition (melting point: 135.degree.
C., crystal dispersion temperature: 100.degree. C., Mw:
6.8.times.10.sup.5, and Mw/Mn: 20.0) comprising 20% by mass of
UHMWPE and 80% by mass of HDPE was used, and that the resin
concentration was 30% by mass. The resin solution was extruded from
a T-die attached to a tip end of a double-screw extruder, and
cooled while drawing by a cooling roll controlled at 0.degree. C.,
thereby providing a gel-like sheet. The gel-like sheet was
simultaneously and biaxially stretched to 5-fold in both
longitudinal direction (MD) and transverse direction (TD) at
115.degree. C. by a tenter-stretching machine. The stretched
gel-like sheet was washed and air-dried in the same manner as in
Example 1. Fixed to a tenter, the dried membrane was heat-set at
125.degree. C. for 10 minutes to produce a microporous polyethylene
membrane.
COMPARATIVE EXAMPLE 2
[0225] Two resin solutions were prepared in the same manner as in
Example 1 except for using resin concentrations of 30% by mass and
28% by mass, respectively. A microporous polyethylene membrane was
produced in the same manner as in Example 1, except that the above
resin solutions were used, that simultaneous biaxial stretching was
conducted at 115.degree. C., that the three-layer, gel-like sheet
was washed and then stretched to 1.4-fold in TD at 124.degree. C.,
and that the heat-setting temperature was 124.degree. C.
TABLE-US-00001 TABLE 1 No. Example 1 Example 2 Example 3 Example 4
Composition of Resin Resin Composition A UHMWPE Mw.sup.(1)/% by
mass 2.0 .times. 10.sup.6/18 2.0 .times. 10.sup.6/18 2.0 .times.
10.sup.6/35 2.0 .times. 10.sup.6/35 HDPE Mw.sup.(1)/% by mass 3.5
.times. 10.sup.5/82 3.5 .times. 10.sup.5/82 3.5 .times. 10.sup.5/65
3.5 .times. 10.sup.5/65 Heat-Resistant Resin Type -- -- -- --
Mw.sup.(1)/% by mass --/-- --/-- --/-- --/-- PE composition
Mw.sup.(1) 6.4 .times. 10.sup.5 6.4 .times. 10.sup.5 9.3 .times.
10.sup.5 9.3 .times. 10.sup.5 Mw/Mn.sup.(2) 21.0 21.0 24.5 24.5
Melting Point (.degree. C.) 135 135 135 135 Crystal Dispersion
Temp. (.degree. C.) 100 100 100 100 Resin Composition B UHMWPE
Mw.sup.(1)/% by mass 2.0 .times. 10.sup.6/18 2.0 .times.
10.sup.6/18 2.0 .times. 10.sup.6/18 2.0 .times. 10.sup.6/18 HDPE
Mw.sup.(1)/% by mass 3.5 .times. 10.sup.5/82 3.5 .times.
10.sup.5/82 3.5 .times. 10.sup.5/82 3.5 .times. 10.sup.5/82
Heat-Resistant Resin Type -- -- -- -- Mw.sup.(1)/% by mass --/--
--/-- --/-- --/-- PE composition Mw.sup.(1) 6.4 .times. 10.sup.5
6.4 .times. 10.sup.5 6.4 .times. 10.sup.5 6.4 .times. 10.sup.5
Mw/Mn.sup.(2) 21.0 21.0 21.0 21.0 Melting Point (.degree. C.) 135
135 135 135 Crystal Dispersion Temp. (.degree. C.) 100 100 100 100
Production Conditions Conc. (% by mass) of A and B.sup.(3) .sup.
40/20.sup.(4) 40/20 40/15 40/15 Extrudate Layer Structure.sup.(5)
B/A/B B/A/B A/B/A A/B/A Layer Thickness Ratio 1/1/1 1/1/1 1/1/1
2/1/2 Stretching Multi-Layer, Gel-Like Sheet Temp. (.degree.
C.)/(MD .times. TD).sup.(6) 117.5/5 .times. 5 117.5/5 .times. 5
115/5 .times. 5 115/5 .times. 5 Gel-Like Sheet A Temp. (.degree.
C.)/(MD .times. TD).sup.(6) --/-- --/-- --/-- --/-- Gel-Like Sheet
B Temp. (.degree. C.)/(MD .times. TD).sup.(6) --/-- --/-- --/--
--/-- Heat-Setting of Gel-Like Sheet Temp. (.degree. C.)/Time
(minute) --/-- --/-- --/-- --/-- Hot Solvent Treatment.sup.(7)
Solvent -- -- -- -- Temp. (.degree. C.)/Time (sec.) --/-- --/--
--/-- --/-- Lamination Temp. (.degree. C.)/Pressure (MPa) --/--
--/-- --/-- --/-- Layer Structure.sup.(8) -- -- -- -- Layer
Thickness Ratio -- -- -- -- Re-Stretching Temp. (.degree.
C.)/Direction/Magnification --/--/-- 129/TD/1.4 128.5/TD/1.4
128.5/TD/1.4 Heat-Setting Temp. (.degree. C.)/Time (minute) 128/10
129/10 128.5/10 128.5/10 No. Example 5 Example 6 Example 7 Example
8 Composition of Resin Resin Composition A UHMWPE Mw.sup.(1)/% by
mass 2.0 .times. 10.sup.6/5 2.0 .times. 10.sup.6/5 2.0 .times.
10.sup.6/5 2.0 .times. 10.sup.6/35 HDPE Mw.sup.(1)/% by mass 3.5
.times. 10.sup.5/95 3.5 .times. 10.sup.5/90 3.5 .times. 10.sup.5/90
3.5 .times. 10.sup.5/65 Heat-Resistant Resin Type -- PP PBT --
Mw.sup.(1)/% by mass --/-- 5.3 .times. 10.sup.5/5 3.8 .times.
10.sup.4/5 --/-- PE composition Mw.sup.(1) 4.3 .times. 10.sup.5 4.4
.times. 10.sup.5 4.4 .times. 10.sup.5 9.3 .times. 10.sup.5
Mw/Mn.sup.(2) 16.0 16.0 16.0 24.5 Melting Point (.degree. C.) 135
135 135 135 Crystal Dispersion Temp. (.degree. C.) 100 100 100 100
Resin Composition B UHMWPE Mw.sup.(1)/% by mass 2.0 .times.
10.sup.6/5 2.0 .times. 10.sup.6/5 2.0 .times. 10.sup.6/5 2.0
.times. 10.sup.6/18 HDPE Mw.sup.(1)/% by mass 3.5 .times.
10.sup.5/95 3.5 .times. 10.sup.5/90 3.5 .times. 10.sup.5/90 3.5
.times. 10.sup.5/82 Heat-Resistant Resin Type -- PP PBT --
Mw.sup.(1)/% by mass --/-- 5.3 .times. 10.sup.5/5 3.8 .times.
10.sup.4/5 --/-- PE composition Mw.sup.(1) 4.3 .times. 10.sup.5 4.4
.times. 10.sup.5 4.4 .times. 10.sup.5 6.4 .times. 10.sup.5
Mw/Mn.sup.(2) 16.0 16.0 16.0 21.0 Melting Point (.degree. C.) 135
135 135 135 Crystal Dispersion Temp. (.degree. C.) 100 100 100 100
Production Conditions Conc. (% by mass) of A and B.sup.(3) 40/20
40/20 40/20 40/15 Extrudate Layer Structure.sup.(5) A/B/A A/B/A
A/B/A A/B/A Layer Thickness Ratio 1/1/1 1/1/1 1/1/1 1/1/1
Stretching Multi-Layer, Gel-Like Sheet Temp. (.degree. C.)/(MD
.times. TD).sup.(6) 117.5/5 .times. 5 117.5/5 .times. 5 117.5/5
.times. 5 115/5 .times. 5 Gel-Like Sheet A Temp. (.degree. C.)/(MD
.times. TD).sup.(6) --/-- --/-- --/-- --/-- Gel-Like Sheet B Temp.
(.degree. C.)/(MD .times. TD).sup.(6) --/-- --/-- --/-- --/--
Heat-Setting of Gel-Like Sheet Temp. (.degree. C.)/Time (minute)
--/-- --/-- --/-- 122/10 Hot Solvent Treatment.sup.(7) Solvent --
-- -- -- Temp. (.degree. C.)/Time (sec.) --/-- --/-- --/-- --/--
Lamination Temp. (.degree. C.)/Pressure (MPa) --/-- --/-- --/--
--/-- Layer Structure.sup.(8) -- -- -- -- Layer Thickness Ratio --
-- -- -- Re-Stretching Temp. (.degree. C.)/Direction/Magnification
129/TD/1.4 129/TD/1.4 129/TD/1.4 129.5/TD/1.4 Heat-Setting Temp.
(.degree. C.)/Time (minute) 129/10 129/10 129/10 129.5/10 No.
Example 9 Example 10 Example 11 Example 12 Composition of Resin
Resin Composition A UHMWPE Mw.sup.(1)/% by mass 2.0 .times.
10.sup.6/35 2.0 .times. 10.sup.6/5 2.0 .times. 10.sup.6/30 2.0
.times. 10.sup.6/30 HDPE Mw.sup.(1)/% by mass 3.5 .times.
10.sup.5/65 3.5 .times. 10.sup.5/95 3.5 .times. 10.sup.5/70 3.5
.times. 10.sup.5/70 Heat-Resistant Resin Type -- -- -- --
Mw.sup.(1)/% by mass --/-- --/-- --/-- --/-- PE composition
Mw.sup.(1) 9.3 .times. 10.sup.5 4.3 .times. 10.sup.5 8.2 .times.
10.sup.5 8.2 .times. 10.sup.5 Mw/Mn.sup.(2) 24.5 16.0 23.5 23.5
Melting Point (.degree. C.) 135 135 135 135 Crystal Dispersion
Temp. (.degree. C.) 100 100 100 100 Resin Composition B UHMWPE
Mw.sup.(1)/% by mass 2.0 .times. 10.sup.6/18 2.0 .times.
10.sup.6/18 2.0 .times. 10.sup.6/18 2.0 .times. 10.sup.6/18 HDPE
Mw.sup.(1)/% by mass 3.5 .times. 10.sup.5/82 3.5 .times.
10.sup.5/82 3.5 .times. 10.sup.5/82 3.5 .times. 10.sup.5/82
Heat-Resistant Resin Type -- -- -- -- Mw.sup.(1)/% by mass --/--
--/-- --/-- --/-- PE composition Mw.sup.(1) 6.4 .times. 10.sup.5
6.4 .times. 10.sup.5 6.4 .times. 10.sup.5 6.4 .times. 10.sup.5
Mw/Mn.sup.(2) 21.0 21.0 21.0 21.0 Melting Point (.degree. C.) 135
135 135 135 Crystal Dispersion Temp. (.degree. C.) 100 100 100 100
Production Conditions Conc. (% by mass) of A and B.sup.(3) 40/15
40/20 30/15 30/15 Extrudate Layer Structure.sup.(5) A/B/A -- A/B --
Layer Thickness Ratio 1/1/1 -- 1/1 -- Stretching Multi-Layer,
Gel-Like Sheet Temp. (.degree. C.)/(MD .times. TD).sup.(6) 115/5
.times. 5 --/-- 119.2/5 .times. 5 --/-- Gel-Like Sheet A Temp.
(.degree. C.)/(MD .times. TD).sup.(6) --/-- 116/5 .times. 5 --/--
119.2/5 .times. 5 Gel-Like Sheet B Temp. (.degree. C.)/(MD .times.
TD).sup.(6) --/-- 116/5 .times. 5 --/-- 119.2/5 .times. 5
Heat-Setting of Gel-Like Sheet Temp. (.degree. C.)/Time (minute)
--/-- --/-- --/-- --/-- Hot Solvent Treatment.sup.(7) Solvent
LP.sup.(9) -- -- -- Temp. (.degree. C.)/Time (sec.) 120/3 --/--
--/-- --/-- Lamination Temp. (.degree. C.)/Pressure (MPa) --/--
110/0.05 --/-- 110/0.05 Layer Structure.sup.(8) -- A/B -- A/B Layer
Thickness Ratio -- 1/1 -- 1/1 Re-Stretching Temp. (.degree.
C.)/Direction/Magnification 130/TD/1.4 --/--/-- 110/TD/1.4
110/TD/1.4 Heat-Setting Temp. (.degree. C.)/Time (minute) 130/10
126/10 110/10 120/10 No. Example 13 Example 14 Comp. Ex. 1 Comp.
Ex. 2 Composition of Resin Resin Composition A UHMWPE Mw.sup.(1)/%
by mass 2.0 .times. 10.sup.6/5 2.0 .times. 10.sup.6/5 2.0 .times.
10.sup.6/20 2.0 .times. 10.sup.6/18 HDPE Mw.sup.(1)/% by mass 3.5
.times. 10.sup.5/95 3.5 .times. 10.sup.5/95 3.5 .times. 10.sup.5/80
3.5 .times. 10.sup.5/82 Heat-Resistant Resin Type -- -- -- --
Mw.sup.(1)/% by mass --/-- --/-- --/-- --/-- PE composition
Mw.sup.(1) 4.3 .times. 10.sup.5 4.3 .times. 10.sup.5 6.8 .times.
10.sup.5 6.4 .times. 10.sup.5 Mw/Mn.sup.(2) 16.0 16.0 20.0 21.0
Melting Point (.degree. C.) 135 135 135 135 Crystal Dispersion
Temp. (.degree. C.) 100 100 100 100 Resin Composition B UHMWPE
Mw.sup.(1)/% by mass 2.0 .times. 10.sup.6/18 2.0 .times.
10.sup.6/18 --/-- 2.0 .times. 10.sup.6/18 HDPE Mw.sup.(1)/% by mass
3.5 .times. 10.sup.5/82 3.5 .times. 10.sup.5/82 --/-- 3.5 .times.
10.sup.5/82 Heat-Resistant Resin Type -- -- -- -- Mw.sup.(1)/% by
mass --/-- --/-- --/-- --/-- PE composition Mw.sup.(1) 6.4 .times.
10.sup.5 6.4 .times. 10.sup.5 -- 6.4 .times. 10.sup.5 Mw/Mn.sup.(2)
21.0 21.0 -- 21.0 Melting Point (.degree. C.) 135 135 -- 135
Crystal Dispersion Temp. (.degree. C.) 100 100 -- 100 Production
Conditions Conc. (% by mass) of A and B.sup.(3) 30/20 30/20 30/--
30/28 Extrudate Layer Structure.sup.(5) A/B A/B/A Layer Thickness
Ratio 1/1 -- -- 1/1/1 Stretching Multi-Layer, Gel-Like Sheet Temp.
(.degree. C.)/(MD .times. TD).sup.(6) --/-- --/-- --/-- 115/5
.times. 5 Gel-Like Sheet A Temp. (.degree. C.)/(MD .times.
TD).sup.(6) --/-- --/-- 115/5 .times. 5 --/-- Gel-Like Sheet B
Temp. (.degree. C.)/(MD .times. TD).sup.(6) --/-- --/-- --/-- --/--
Heat-Setting of Gel-Like Sheet Temp. (.degree. C.)/Time (minute) --
-- -- -- Hot Solvent Treatment.sup.(7) Solvent -- -- -- -- Temp.
(.degree. C.)/Time (sec.) --/-- --/-- --/-- --/-- Lamination Temp.
(.degree. C.)/Pressure (MPa) --/-- 110/0.05 --/-- --/-- Layer
Structure.sup.(8) -- A/B -- -- Layer Thickness Ratio -- 1/1 -- --
Re-Stretching Temp. (.degree. C.)/Direction/Magnification --/--/--
--/--/-- --/--/-- 124/TD/1.4 Heat-Setting Temp. (.degree. C.)/Time
(minute) 125/10 128/10 125/10 124/10 Note: .sup.(1)Mw represents a
mass-average molecular weight. .sup.(2)Mw/Mn represents a molecular
weight distribution. .sup.(3)The concentrations of the resin
compositions in the resin solutions A and B, respectively.
.sup.(4)The resin concentration in the resin solution A/the resin
concentration in the resin solution B. .sup.(5)A represents a resin
solution A, and B represents a resin solution B.
.sup.(6)Temperature (.degree. C.)/stretching magnification (fold)
in MD and TD, wherein MD represents a longitudinal direction, and
TD represents a transverse direction. .sup.(7)The heat-setting of a
gel-like sheet. .sup.(8)A represents a microporous membrane A, and
B represents a
microporous membrane B. .sup.(9)LP represents liquid paraffin.
[0226] The properties of the microporous polyethylene membranes
obtained in Examples 1 to 14 and Comparative Examples 1 and 2 were
measured by the following methods. The results are shown in Table
2.
[0227] (1) Average Thickness (.mu.m)
[0228] The thickness of the microporous polyethylene membrane was
measured at a 5-mm interval over a width of 30 cm by a contact
thickness meter, and the measured thickness was averaged.
[0229] (2) Air Permeability (sec/100 cm.sup.3/20 .mu.m)
[0230] The air permeability P.sub.1 of the microporous polyethylene
membrane having a thickness T.sub.1 was measured according to JIS
P8117, and converted to air permeability P.sub.2 at a thickness of
20 .mu.m by the formula of P.sub.2=(P.sub.1.times.20)/T.sub.1.
[0231] (3) Porosity (%)
[0232] It was measured by a mass method.
[0233] (4) Pin Puncture Strength (mN/20 .mu.m)
[0234] The maximum load was measured when a microporous
polyethylene membrane having a thickness T.sub.1 was pricked with a
needle of 1 mm in diameter with a spherical end surface (radius R
of curvature: 0.5 mm) at a speed of 2 mm/second. The measured
maximum load L.sub.1 was converted to the maximum load L.sub.2 at a
thickness of 20 .mu.m by the formula of
L.sub.2=(L.sub.1.times.20)/T.sub.1, which was regarded as pin
puncture strength.
[0235] (5) Tensile Rupture Strength and Tensile Rupture
Elongation
[0236] They were measured using a 10-mm-wide rectangular test piece
according to ASTM D882.
[0237] (6) Heat Shrinkage Ratio (%)
[0238] The shrinkage ratio of the microporous polyethylene membrane
after exposed to 105.degree. C. for 8 hours was measured three
times in both longitudinal direction (MD) and transverse direction
(TD) and averaged.
[0239] (7) Shutdown Temperature
[0240] Using a thermomechanical analyzer (TMA/SS6000 available from
Seiko Instruments, Inc.), a test piece of 10 mm (TD).times.3 mm
(MD) was heated at a speed of 5.degree. C./minute from room
temperature while being longitudinally drawn under a load of 2 g. A
temperature at an inflection point observed near the melting point
was regarded as a shutdown temperature.
[0241] (8) Meltdown Temperature (.degree. C.)
[0242] Using the above thermomechanical analyzer, a test piece of
10 mm (TD).times.3 mm (MD) was heated from room temperature at a
speed of 5.degree. C./minute while longitudinally drawing by a load
of 2 g, to measure a temperature at which the membrane was broken
by melting.
[0243] (9) Ratio of Thickness Change by Heat Compression
[0244] A microporous membrane sample was sandwiched by a pair of
press plates each having a highly smooth surface, and
heat-compressed by a press machine at a pressure of 2.2 MPa (22
kgf/cm.sup.2) and 90.degree. C. for 5 minutes to measure the
average thickness by the above method. A thickness change ratio was
calculated, assuming that the pre-compression thickness was
100%.
[0245] (10) Post-Compression Air Permeability (sec/100
cm.sup.3)
[0246] The air permeability of the microporous polyethylene
membrane heat-compressed under the above conditions was measured
according to JIS P8117, and regarded as "post-compression air
permeability."
[0247] (11) Average Pore Diameter
[0248] 50 pores were arbitrarily selected in each microporous layer
A, B in a transmission electron photmicrograph (TEM photograph) of
a cross section of the microporous membrane, and their sizes were
measured and averaged to determine an average pore diameter in each
layer.
TABLE-US-00002 TABLE 2 No. Example 1 Example 2 Example 3 Example 4
Properties of Microporous Membrane Average Thickness (.mu.m) 28.2
21.5 20.8 22.4 Air Permeability (sec/100 cm.sup.3/20 .mu.m) 315 210
275 347 Porosity (%) 43.6 47.8 48.1 48.1 Pin Puncture Strength
(g/20 .mu.m) 480 520 612 668 (mN/20 .mu.m) 4,704 5,096 5,998 6,546
Tensile Rupture Strength (kg/cm.sup.2, kPa) MD 1,350/132,300
1,480/145,040 1,670/163,660 1,750/171,500 TD 1,150/112,700
1,470/144,060 1,560/152,880 1,790/175,420 Tensile Rupture
Elongation (%) MD/TD 150/280 170/180 140/120 135/130 Heat Shrinkage
Ratio (%) MD/TD 5/4 4/6 6/8 6.5/8 Thermal Properties Shutdown
Temperature (.degree. C.) 135 140 140 140 Meltdown Temperature
(.degree. C.) 165 165 165 165 Compression Resistance Thickness
Change (%) 22 24 16 14 Post-Compression Air Permeability 620 510
630 760 (sec/100 cm.sup.3/20 .mu.m) Average Pore Diameter
(.mu.m).sup.(1) Microporous Layer A 0.03 (Inner) 0.04 (Inner) 0.04
(Surface) 0.04 (Surface) Microporous Layer B 0.06 (Surface) 0.09
(Surface) 0.08 (Inner) 0.08 (Inner) No. Example 5 Example 6 Example
7 Example 8 Properties of Microporous Membrane Average Thickness
(.mu.m) 21 21 21 20.5 Air Permeability (sec/100 cm.sup.3/20 .mu.m)
210 190 180 212 Porosity (%) 40 38 37 43 Pin Puncture Strength
(g/20 .mu.m) 550 490 460 590 (mN/20 .mu.m) 5,390 4,802 4,508 5,782
Tensile Rupture Strength (kg/cm.sup.2, kPa) MD 1,340/131,320
1,180/115,640 1,020/99,960 1,505/147,490 TD 1,330/130,340
1,200/117,600 1,090/106,820 1,510/147,980 Tensile Rupture
Elongation (%) MD/TD 170/160 160/140 140/130 160/145 Heat Shrinkage
Ratio (%) MD/TD 4/6 3/4 3/4 5/5 Thermal Properties Shutdown
Temperature (.degree. C.) 140 140 140 140 Meltdown Temperature
(.degree. C.) 160 175 175 165 Compression Resistance Thickness
Change (%) 18 24 28 19 Post-Compression Air Permeability 480 450
410 510 (sec/100 cm.sup.3/20 .mu.m) Average Pore Diameter
(.mu.m).sup.(1) Microporous Layer A 0.07 (Surface) 0.09 (Surface)
0.09 (Surface) 0.05 (Surface) Microporous Layer B 0.1 (Inner) 0.1
(Inner) 0.1 (Inner) 0.09 (Inner) No. Example 9 Example 10 Example
11 Example 12 Properties of Microporous Membrane Average Thickness
(.mu.m) 22 24.3 16 30.5 Air Permeability (sec/100 cm.sup.3/20
.mu.m) 196 470 50 110 Porosity (%) 48.1 42.1 63 60.5 Pin Puncture
Strength (g/20 .mu.m) 560 580 220 410 (mN/20 .mu.m) 5,488 5,684
2,156 4,018 Tensile Rupture Strength (kg/cm.sup.2, kPa) MD
1,450/142,100 1,450/142,100 950/93,100 850/83,300 TD 1,490/146,020
1,200/117,600 780/76,440 940/92,120 Tensile Rupture Elongation (%)
MD/TD 160/150 170/180 170/140 160/140 Heat Shrinkage Ratio (%)
MD/TD 5/4 2/0 13/15 12/16 Thermal Properties Shutdown Temperature
(.degree. C.) 140 135 135 135 Meltdown Temperature (.degree. C.)
165 165 165 165 Compression Resistance Thickness Change (%) 22 19
27 25 Post-Compression Air Permeability 490 890 213 250 (sec/100
cm.sup.3/20 .mu.m) Average Pore Diameter (.mu.m).sup.(1)
Microporous Layer A 0.06 (Surface) 0.03 0.05 0.05 Microporous Layer
B 0.1 (Inner) 0.05 0.07 0.07 No. Example 13 Example 14 Comp. Ex. 1
Comp. Ex. 2 Properties of Microporous Membrane Average Thickness
(.mu.m) 60 60 16 22 Air Permeability (sec/100 cm.sup.3/20 .mu.m) 35
40 400 445 Porosity (%) 64.8 65.2 38 40 Pin Puncture Strength (g/20
.mu.m) 120 110 400 520 (mN/20 .mu.m) 1,176 1,078 3,920 5,096
Tensile Rupture Strength (kg/cm.sup.2, kPa) MD 180/17,640
200/19,600 1,400/137,200 1,240/121,520 TD 140/13,720 150/14,700
1,200/117,600 1,200/117,600 Tensile Rupture Elongation (%) MD/TD
190/180 200/180 145/230 170/160 Heat Shrinkage Ratio (%) MD/TD
27/18 25/20 6/4 5/6 Thermal Properties Shutdown Temperature
(.degree. C.) 130 130 135 135 Meltdown Temperature (.degree. C.)
160 160 160 165 Compression Resistance Thickness Change (%) 30 30
18 16 Post-Compression Air Permeability 120 150 1,060 1,070
(sec/100 cm.sup.3/20 .mu.m) Average Pore Diameter (.mu.m).sup.(1)
Microporous Layer A 0.1 0.05 0.03 0.03 (Surface) Microporous Layer
B 0.2 0.2 0.03 0.03 (Inner) Note: .sup.(1)Surface represents a
surface layer, and Inner represents an inner layer.
[0249] It is clear from Table 2 that because each microporous
polyethylene membrane of Examples 1 to 14 produced by the method of
this invention had a gradient structure in which an average pore
diameter changed in a thickness direction, it had excellent
compression resistance (deformability when compressed and
permeability after compression), permeability, mechanical
properties, heat shrinkage resistance and thermal properties.
[0250] On the other hand, one resin solution was used to form the
gel-like sheet in Comparative Example 1, and two resin solutions
used to form the three-layer, gel-like sheet in Comparative Example
2 had a resin concentration difference of less than 5% by mass.
Accordingly, any membranes of Comparative Examples 1 and 2 had
larger post-compression air permeability and poorer compression
resistance than those in Examples 1 to 14.
EFFECT OF THE INVENTION
[0251] According to this invention, a microporous polyethylene
membrane with an average pore diameter changing in a thickness
direction, which has well-balanced permeability, mechanical
properties, heat shrinkage resistance, compression resistance,
electrolytic solution absorbability, shutdown properties and
meltdown properties, can be produced while easily controlling an
average pore diameter distribution in a thickness direction. It is
easy to control the ratio of the coarse-structure layer having a
larger average pore diameter to the dense-structure layer having a
smaller average pore diameter, and the pore size in each layer. The
use of the microporous polyethylene membrane produced by the method
of this invention as a battery separator provides batteries with
excellent capacity, cycle properties, discharge properties, heat
resistance, compression resistance and productivity.
* * * * *